Sherpa function to facilitate loading HETG responses associated with a PHA2 file

[dprincipe2]

If users generated or obtained an HETG PHA2 file through relatively standard means (e.g., Chandra archive or running chandra_repro) then this function will load their PHA2 file into sherpa and find/assign the associated ARFs and RMFs.

[DougBurke]

I suggest moving the two local routines - find_resp_files and match_resp_order - to the top level as it looks like (not 100% convinced) they don't rely on any specialized knowledge provided by load_hetg_pha2. Splitting them out makes them easier to test and the code a little easier to follow.

Looking at the code, I see there's a number of cases where you have something like

for i in range(len(some_array)):
   ... do something with some_array[i] ...

This can often be written as

for val in some_array:
   ... do something with val ...

which I personally find easier to read (this is not always the case; it depends on whether you need the index for anything other than grabbing the value from a container like some_array).

I tried this with a HETG dataset I grabbed from TGCat but it raised an error, so I'm either using it wrong or it doesn't handle the TGCat case"

sherpa-4.18.0+2.ga0c5211a> load_hetg_pha2("pha2.gz")
ValueError: Could not identify PHA2 file root. Please load responses manually.

[dprincipe2]

Sure, I can do those things. I tend to loop over ranges because many times I need the index but I can change that for this code for readability. I haven't tested this for tgcat- or tgextract-generated pha2 files but those can probably be added easy enough. Currently it's for chandra repro and the standard archive files.