WIP: Add new distributions (HyperSphericalUniform and PowerSpherical)

[marcpabst]

The Power Spherical distribution (https://arxiv.org/abs/2006.04437) is nice (and simpler!) alternative to the von-Mises-Fisher distribution.

• [ ] add / extend docstrings
• [ ] extend tests
• [x] add known KL divergences

[codecov-commenter]

Codecov Report

Merging #1594 (fc2f53f) into master (0c9367c) will decrease coverage by 0.90%. The diff coverage is 1.63%.

@@            Coverage Diff             @@
##           master    #1594      +/-   ##
==========================================
- Coverage   85.73%   84.82%   -0.91%     
==========================================
  Files         129      130       +1     
  Lines        7971     7995      +24     
==========================================
- Hits         6834     6782      -52     
- Misses       1137     1213      +76     

Impacted Files	Coverage Δ
src/multivariate/hypersphericaluniform.jl	0.00% <0.00%> (ø)
src/multivariate/powerspherical.jl	0.00% <0.00%> (ø)
src/multivariates.jl	38.46% <ø> (-4.40%)	:arrow_down:
src/samplers/powerspherical.jl	0.00% <0.00%> (ø)
src/multivariate/vonmisesfisher.jl	93.33% <100.00%> (ø)
src/utils.jl	64.40% <0.00%> (-28.02%)	:arrow_down:
src/univariates.jl	71.42% <0.00%> (-2.65%)	:arrow_down:
src/matrixvariates.jl	95.23% <0.00%> (-0.42%)	:arrow_down:
src/common.jl	80.41% <0.00%> (-0.20%)	:arrow_down:
... and 3 more

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