Inverse Gamma Node

[abpolym]

Adds the inverse gamma node/distribution.

[albertpod]

Good job @abpolym! Once @bvdmitri comments are addressed, we can merge!

[abpolym]

I've added the requested changes 😄

[codecov-commenter]

Codecov Report

Merging #194 (ee12855) into master (b00efa9) will increase coverage by 0.10%. The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master     #194      +/-   ##
==========================================
+ Coverage   76.73%   76.84%   +0.10%     
==========================================
  Files         200      203       +3     
  Lines        6676     6703      +27     
==========================================
+ Hits         5123     5151      +28     
+ Misses       1553     1552       -1     

Impacted Files	Coverage Δ
src/ReactiveMP.jl	66.66% <ø> (ø)
src/distributions/gamma_inverse.jl	91.66% <91.66%> (ø)
src/nodes/gamma_inverse.jl	100.00% <100.00%> (ø)
src/rules/gamma_inverse/marginals.jl	100.00% <100.00%> (ø)
src/rules/normal_mean_variance/var.jl	100.00% <100.00%> (ø)
src/rule.jl	54.67% <0.00%> (+0.30%)	:arrow_up:

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