feat: init binned spectrum and add axis conversions

[jamesrafe]

No description provided.

[fjebaker]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.75%. Comparing base (48a80f1) to head (0ab78c9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #61      +/-   ##
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+ Coverage   94.50%   95.75%   +1.24%     
==========================================
  Files           9       11       +2     
  Lines         182      212      +30     
==========================================
+ Hits          172      203      +31     
+ Misses         10        9       -1     

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[icweaver]

This is a really nice PR, thanks @jamesrafe! I think this would be a good opportunity to flex our ecosystem here and start factoring out some common functionality I've been seeing pop-up in a few of our packages.

For starters: @cgarling, what would you think about upstreaming your spectral density conversions in PhotometricFilters.jl to UnitfulEquivalences.jl? I've opened a PR to demonstrate what that might look like sostock/UnitfulEquivalences.jl#31

Next, I think we could replace the custom to_wavelength/to_frequency API introduced in this PR with simple methods extending Untiful.jl's uconvert. Something like this seems ergonomic to me:

uconvert(u"THz", spec) # Just convert the spectral axis
uconvert((u"keV", u"W/m^2/keV"), spec) # Convert the spectral AND flux axis

compared to the separate with_spectral_axis_and_flux_units()/with_spectral_axis_unit()/with_flux_unit() that astropy/specutils seems to juggle

There's also some cleanup in the binned spectrum logic I think we could apply here. This will all be included in my review, but just wanted to give y'all a heads up first on where I was thinking of taking this

[cgarling]

No problem with moving the spectral density conversions if they accept the pr, IMO its a little more specialized than their other equivalences so IDK if they will want to add it

[icweaver]

I may have been spoiled with the DynamicQuantities dev being so receptive to some of our astro-specific PRs in the past, so will see I guess 😅