Improve heteroscedastic

[theogf]

• Add option for sampling
• Improve the Polya-Gamma sampler
• Replace a lot of broadcast by map

[coveralls]

Pull Request Test Coverage Report for Build 1505123595

• 140 of 162 (86.42%) changed or added relevant lines in 11 files are covered.
• 5 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.5%) to 68.762%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/likelihood/matern.jl	0	10	0.0%
src/ComplementaryDistributions/polyagamma.jl	61	73	83.56%
<!--	Total:	140	162	86.42%	-->

Files with Coverage Reduction	New Missed Lines	%
src/ComplementaryDistributions/polyagamma.jl	1	85.87%
src/likelihood/laplace.jl	1	90.0%
src/likelihood/logistic.jl	1	88.0%
src/likelihood/matern.jl	2	0%
<!--	Total:	5		-->

Totals
Change from base Build 1494156205:	0.5%
Covered Lines:	1849
Relevant Lines:	2689

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💛 - Coveralls