feat: add InverseGamma natural manifold

[Nimrais]

This PR adds InverseGamma natural manifold

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.63%. Comparing base (5960f56) to head (a2c6ded).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #18      +/-   ##
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+ Coverage   96.59%   96.63%   +0.04%     
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  Files          24       25       +1     
  Lines         323      327       +4     
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+ Hits          312      316       +4     
  Misses         11       11              

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[bvdmitri]

Looks fine to me, but its a bit weird that we need to prepend it with ExponentialFamily, since

julia> using ExponentialFamily

julia> using Distributions

julia> ExponentialFamily.InverseGamma === Distributions.InverseGamma
true

julia> ExponentialFamily.GammaInverse === Distributions.InverseGamma
true

[Nimrais]

Looks fine to me, but its a bit weird that we need to prepend it with ExponentialFamily, since

julia> using ExponentialFamily

julia> using Distributions

julia> ExponentialFamily.InverseGamma === Distributions.InverseGamma
true

julia> ExponentialFamily.GammaInverse === Distributions.InverseGamma
true

Yeah, it's true. However it's because we do not have Distributions.jl in our dependencies and ExponentialFamily does not export InverseGamma or GammaInverse.