add spat unit of solid angle

[ntessore]

Description

This pull request is to address #10721. It adds the spat unit (abbreviation sp, apparently) where 1 spat corresponds to 4pi steradian, i.e. the solid angle of the entire sphere.

That makes it possible to convert fsky sky fractions to solid angle transparently:

survey_area = fsky*u.spat

And also to convert solid angles to sky fractions:

sky = survey_area.to_value(u.spat)

Fixes #10721

[pllim]

Hello and thank you! Please add a change log under "new features" under the given milestone.

@mhvk , does this need a "what's new"?

[mhvk]

Super! And funny to see that in the previous PR for cycle, we didn't seem to see a need to include it even in CHANGES.rst - we have made some progress! Don't think it is necessary to add to what's new - I hope units are such that when people want something, it is already there, and, if not, they'll add it (like here). Thanks, @ntessore!

[ntessore]

Thanks everyone for the quick help! That was fun