WIP: Multivariate factors

[abraunst]

doctests probably failing....

[codecov]

Codecov Report

Merging #9 into master will decrease coverage by 16.69%.
The diff coverage is 33.78%.

@@            Coverage Diff             @@
##           master       #9      +/-   ##
==========================================
- Coverage   45.39%   28.69%   -16.7%     
==========================================
  Files           5        8       +3     
  Lines         163      223      +60     
==========================================
- Hits           74       64      -10     
- Misses         89      159      +70

Impacted Files	Coverage Δ
src/GaussianEP.jl	100% <ø> (ø)	⬆️
src/multivariate.jl	0% <0%> (ø)
src/legacy.jl	0% <0%> (ø)
src/FactorGraph.jl	100% <100%> (ø)
src/univariate.jl	20.95% <27.5%> (ø)
src/Factor.jl	33.33% <33.33%> (ø)
src/expectation_propagation.jl	92.68% <92.3%> (+2.88%)	⬆️
... and 1 more

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