Introduce index-based functions for system interactions so that we can handle intersystem specific interactions

[svchb]

based on #1056

[codecov]

Codecov Report

❌ Patch coverage is 76.28205% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.55%. Comparing base (3efbc96) to head (4d8c5a7).

Files with missing lines	Patch %	Lines
...chemes/fluid/implicit_incompressible_sph/system.jl	8.33%	11 Missing ⚠️
src/general/neighborhood_search.jl	75.67%	9 Missing ⚠️
src/general/semidiscretization.jl	88.00%	9 Missing ⚠️
src/io/io.jl	0.00%	2 Missing ⚠️
src/schemes/boundary/open_boundary/system.jl	77.77%	2 Missing ⚠️
src/callbacks/split_integration.jl	0.00%	1 Missing ⚠️
src/schemes/fluid/fluid.jl	80.00%	1 Missing ⚠️
...rc/schemes/fluid/weakly_compressible_sph/system.jl	0.00%	1 Missing ⚠️
...c/schemes/structure/total_lagrangian_sph/system.jl	88.88%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (3efbc96) and HEAD (4d8c5a7). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (3efbc96)	HEAD (4d8c5a7)
total	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1057       +/-   ##
===========================================
- Coverage   89.39%   65.55%   -23.84%     
===========================================
  Files         121      121               
  Lines        8866     8850       -16     
===========================================
- Hits         7926     5802     -2124     
- Misses        940     3048     +2108     

Flag	Coverage Δ
total	?
unit	65.55% <76.28%> (+0.15%)	⬆️

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