Ellipse: Do not throw error for theta=0

[pllim]

Description

theta=0 is the same as no rotation, so throwing NotImplementedError for that case is confusing.

[codecov]

Codecov Report

Merging #2280 (519874b) into main (55428b0) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2280      +/-   ##
==========================================
- Coverage   88.09%   88.09%   -0.01%     
==========================================
  Files         247      247              
  Lines       23293    23293              
==========================================
- Hits        20521    20520       -1     
- Misses       2772     2773       +1     

Impacted Files	Coverage Δ
glue/core/roi.py	91.92% <100.00%> (ø)
glue/conftest.py	66.25% <0.00%> (-1.25%)	⬇️

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[dhomeier]

This would be covered by #2235, but that's a somewhat larger PR, so might be preferable to put this in as a temporary fix. Not quite sure why the existing version even has a theta parameter.

[pllim]

Up to you. This patch is not critical for my work. I noticed the error and it didn't make sense to me.

[astrofrog]

I'll merge this as no real downside even though theta shouldn't be set in the first place.