Generalize GaussianLinearCombination to operators

[arnavk23]

This PR fixes #83 by generalizing GaussianLinearCombination to support homogeneous linear combinations of GaussianState, GaussianUnitary, or GaussianChannel (with consistent basis and ħ).
It adds operator/channel arithmetic support (scalar ops and +/- with matching LC types), while keeping state-specific functionality and backward compatibility intact.

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	main	a9d7e54...	main / a9d7e54...
operations/channel product/10	4.64 ± 0.8 μs	4.76 ± 0.69 μs	0.975 ± 0.22
operations/channel product/100	0.68 ± 0.065 ms	0.682 ± 0.072 ms	0.997 ± 0.14
operations/channel product/2	1.3 ± 0.09 μs	1.39 ± 0.11 μs	0.935 ± 0.098
operations/channel product/200	3.51 ± 1.1 ms	3.51 ± 0.75 ms	1 ± 0.39
operations/channel product/50	0.237 ± 0.02 ms	0.242 ± 0.025 ms	0.978 ± 0.13
operations/partial trace/10	1.29 ± 0.13 μs	1.32 ± 0.13 μs	0.977 ± 0.14
operations/partial trace/100	8.56 ± 1.4 μs	8.62 ± 1.2 μs	0.992 ± 0.21
operations/partial trace/2	0.892 ± 0.06 μs	0.902 ± 0.05 μs	0.989 ± 0.086
operations/partial trace/200	22.5 ± 3 μs	23.2 ± 3.3 μs	0.971 ± 0.19
operations/partial trace/50	4.59 ± 0.63 μs	4.58 ± 0.62 μs	1 ± 0.19
operations/tensor product/10	2.31 ± 0.38 μs	2.13 ± 0.41 μs	1.08 ± 0.27
operations/tensor product/100	0.151 ± 0.017 ms	0.148 ± 0.019 ms	1.02 ± 0.17
operations/tensor product/2	0.821 ± 0.059 μs	0.862 ± 0.06 μs	0.952 ± 0.095
operations/tensor product/200	0.817 ± 0.11 ms	0.803 ± 0.18 ms	1.02 ± 0.26
operations/tensor product/50	0.032 ± 0.0064 ms	0.0317 ± 0.0093 ms	1.01 ± 0.36
operations/unitary product/10	3.63 ± 0.29 μs	3.69 ± 0.38 μs	0.984 ± 0.13
operations/unitary product/100	0.643 ± 0.037 ms	0.639 ± 0.026 ms	1.01 ± 0.071
operations/unitary product/2	1.15 ± 0.05 μs	1.17 ± 0.051 μs	0.983 ± 0.06
operations/unitary product/200	3.4 ± 0.11 ms	3.41 ± 0.099 ms	0.997 ± 0.043
operations/unitary product/50	0.22 ± 0.016 ms	0.22 ± 0.016 ms	1 ± 0.1
time_to_load	0.214 ± 0.00094 s	0.216 ± 0.0026 s	0.991 ± 0.013

Memory benchmarks

	main	a9d7e54...	main / a9d7e54...
operations/channel product/10	17 allocs: 10.2 kB	17 allocs: 10.2 kB	1
operations/channel product/100	17 allocs: 0.919 MB	17 allocs: 0.919 MB	1
operations/channel product/2	14 allocs: 0.891 kB	14 allocs: 0.891 kB	1
operations/channel product/200	19 allocs: 3.67 MB	19 allocs: 3.67 MB	1
operations/channel product/50	17 allocs: 0.231 MB	17 allocs: 0.231 MB	1
operations/partial trace/10	14 allocs: 0.672 kB	14 allocs: 0.672 kB	1
operations/partial trace/100	15 allocs: 2.79 kB	15 allocs: 2.79 kB	1
operations/partial trace/2	14 allocs: 0.672 kB	14 allocs: 0.672 kB	1
operations/partial trace/200	15 allocs: 5.04 kB	15 allocs: 5.04 kB	1
operations/partial trace/50	14 allocs: 1.69 kB	14 allocs: 1.69 kB	1
operations/tensor product/10	9 allocs: 13.1 kB	9 allocs: 13.1 kB	1
operations/tensor product/100	10 allocs: 1.22 MB	10 allocs: 1.22 MB	1
operations/tensor product/2	8 allocs: 0.797 kB	8 allocs: 0.797 kB	1
operations/tensor product/200	10 allocs: 4.89 MB	10 allocs: 4.89 MB	1
operations/tensor product/50	9 allocs: 0.307 MB	9 allocs: 0.307 MB	1
operations/unitary product/10	14 allocs: 6.95 kB	14 allocs: 6.95 kB	1
operations/unitary product/100	14 allocs: 0.614 MB	14 allocs: 0.614 MB	1
operations/unitary product/2	12 allocs: 0.688 kB	12 allocs: 0.688 kB	1
operations/unitary product/200	16 allocs: 2.45 MB	16 allocs: 2.45 MB	1
operations/unitary product/50	14 allocs: 0.155 MB	14 allocs: 0.155 MB	1
time_to_load	0.145 k allocs: 11 kB	0.145 k allocs: 11 kB	1

[codecov]

Codecov Report

❌ Patch coverage is 74.35897% with 30 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@8a15976). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/linearcombinations.jl	74.35%	30 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #104   +/-   ##
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  Coverage        ?   96.16%           
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  Files           ?       19           
  Lines           ?     2709           
  Branches        ?        0           
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  Hits            ?     2605           
  Misses          ?      104           
  Partials        ?        0           

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[arnavk23]

@apkille whenever you are free, Andrew please take a look. Thanks!

[apkille]

Thanks for the PR @arnavk23! And sorry for the slow response, I've been quite busy these past few months. I will do my best to review later this week.