Add SpectrumFileProcessor class for automated spectral data conversion and formatting

[krishnatejavedula]

• Implemented SpectrumFileProcessor class to handle energy and flux data conversion for spectrum files not in the MEGAlib spectrum file format.
• Added energy conversion feature: converts frequency (Hz) to energy (keV) and flux from ergs/cm²/sec to ph/cm²/sec/keV using astropy units.
• Implemented data filtering to restrict energy range to 100 keV - 10,000 keV for accurate source injector functionality.
• Reformatted output to ensure compatibility with SpecFromDat class and consistency across cosipy and DCs spectral files.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 75 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.97%. Comparing base (aecb327) to head (6fa5c5e).
⚠️ Report is 120 commits behind head on develop.

Files with missing lines	Patch %	Lines
cosipy/data_io/custom_functions.py	0.00%	75 Missing ⚠️

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Files with missing lines	Coverage Δ
cosipy/data_io/custom_functions.py	0.00% <0.00%> (ø)

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[hiyoneda]

@krishnatejavedula Thanks for this work. Looks good. I have a quick comment. threeml/custom_function.py, to which you added this new class, is used to implement new spectral and spatial functions for astromodels in threeml fitting. I think that your new class can be used more generally. Can you think of a better directory for this class, maybe dataIO?

[israelmcmc]

Thanks @krishnatejavedula. Any updates to this?

• I think what @hiyoneda suggested makes sense. Since there don't seem to be any interactions of SpectrumFileProcessor with 3ML, you can move this class to its own file inside the DataIO module.
• Besides that, the other thing to resolve is to add unit tests to cover this new class.

[Yong2Sheng]

I tested the module and it works well.