Adds moments for inverse gamma distribution

[morganstrom]

returns the mode when the mean is undefined

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[codecov]

Codecov Report

Merging #5199 (414ff4e) into main (90423a9) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5199      +/-   ##
==========================================
+ Coverage   78.12%   78.13%   +0.01%     
==========================================
  Files          88       88              
  Lines       14174    14181       +7     
==========================================
+ Hits        11073    11080       +7     
  Misses       3101     3101              

Impacted Files	Coverage Δ
pymc/distributions/continuous.py	96.57% <100.00%> (+0.02%)	⬆️

[ricardoV94]

Thanks @morganstrom!