Add Half-normal node

[albertpod]

No description provided.

[albertpod]

ping @bvdmitri

[albertpod]

@bvdmitri note that there are no tests for prod of HalfNormal and Gamma, besides, I can't figure out why check_type_promotion = true fails.

I've also fixed the namings in InverseGamma tests.

[codecov]

Codecov Report

Patch coverage: 73.91% and project coverage change: +0.01 🎉

Comparison is base (5c9ca66) 67.44% compared to head (a234d1b) 67.46%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #338      +/-   ##
==========================================
+ Coverage   67.44%   67.46%   +0.01%     
==========================================
  Files         210      210              
  Lines        7660     7683      +23     
==========================================
+ Hits         5166     5183      +17     
- Misses       2494     2500       +6     

Impacted Files	Coverage Δ
src/ReactiveMP.jl	63.15% <ø> (ø)
src/distributions/normal.jl	90.99% <0.00%> (-0.59%)	⬇️
src/distributions/gamma_inverse.jl	85.71% <75.00%> (-6.60%)	⬇️
src/distributions/gamma.jl	90.24% <84.61%> (-1.07%)	⬇️

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[bvdmitri]

The fail in check_type_promotion is probably due to the Inf which is of type Float64

[albertpod]

ping @bvdmitri