gh-509: inv_triangle_number -> nfields_from_nspectra (plus make it public)

[Saransh-cpp]

Description

Renamed inv_triangle_number -> nfields_from_nspectra
Made the function public and added it to __all__.

Closes: #509

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[paddyroddy]

Lol I was literally just about to raise a PR

[ntessore]

I would update the argument and docstring here to reflect the new function name. Something along the lines of

Returns the number of fields for a number of spectra.

Given the number of spectra nspectra, returns the number of fields n such that n * (n + 1) // 2 == nspectra.

In addition, I would also update the error message:

invalid number of spectra: {nspectra}

That means there's no need to catch and rethrow the error elsewhere in fields.py

[Saransh-cpp]

Updated!

[Saransh-cpp]

Thanks for the fixes, @ntessore!