Add confirm_recomp option to IsochronePhot; fix PYSYN_CDBS import-time lookup

[wei-lingfeng]

Three small, independent changes, one per commit.

1. confirm_recomp option for IsochronePhot

IsochronePhot.__init__ currently prompts before regenerating an isochrone file whenever verbose=True. It accepts only the exact string "yes" and returns on anything else, so a typo silently cancels the run.

This puts the prompt behind a new confirm_recomp argument and loops on invalid input instead of cancelling.

Please note this changes existing behaviour: confirm_recomp defaults to False, so a verbose=True run that prompts today will no longer prompt. That seemed right to us — the prompt makes an otherwise scriptable call block on stdin — but happy to default it to True instead if you'd rather preserve the current behaviour.

Also documents confirm_recomp and the pre-existing verbose argument, and splits the long __init__ signature across lines.

2. Remove trailing whitespace in synthetic.py

Whitespace only. git diff --ignore-space-at-eol on this commit is empty. Split out so it doesn't obscure the change above; drop it if you'd prefer to keep it out.

3. Resolve PYSYN_CDBS at call time rather than import time

rebin_BTSettl_2015 and rebin_WDKoester read os.environ['PYSYN_CDBS'] in their default arguments, which Python evaluates when the module is imported. So import spisea.atmospheres raises KeyError: 'PYSYN_CDBS' on any machine where that variable is not set, even for code that never calls either function:

File "spisea/atmospheres.py", line 2420, in <module>
    def rebin_BTSettl_2015(cdbs_path=os.environ['PYSYN_CDBS']):
KeyError: 'PYSYN_CDBS'

Moving the lookup into the function bodies behind a None default fixes it. Callers with PYSYN_CDBS set see identical behaviour, and an explicit cdbs_path still overrides. The other PYSYN_CDBS references in the file are inside function bodies and were already fine.

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Related: the API docs for synthetic are currently blank on RTD

While tracking the above down we noticed that every synthetic.* class section on Read the Docs renders as an empty heading — make_isochrone and make_cluster — while imf and evolution render fine. autodoc cannot import synthetic in the RTD build, and there are three sequential import-time blockers:

1. atmospheres.py PYSYN_CDBS in default args — fixed by this PR
2. synthetic.py:42 default_evo_model = evolution.MISTv1() — requires the MISTv1/v1.2-synthpop/ directory to exist
3. synthetic.py:68 vega = Vega() — reads cdbs/grid/k93models/catalog.fits

So this PR clears only the first. 2 and 3 need the model data at import time, which the RTD builder will not have. We didn't attempt those here since making the module importable without data is a design decision for you — happy to open a separate issue with the details if that's useful.

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