Fix dispatch with likelihood and make ForwardDiff work for any kernel

[theogf]

• make ForwardDiff work and test for it
• Better dispatch for the likelihood function

[coveralls]

Pull Request Test Coverage Report for Build 673

• 70 of 121 (57.85%) changed or added relevant lines in 24 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.9%) to 57.115%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/functions/utils.jl	0	1	0.0%
src/likelihood/studentt.jl	1	2	50.0%
src/training/predictions.jl	2	3	66.67%
src/likelihood/bayesiansvm.jl	1	3	33.33%
src/likelihood/logistic.jl	3	5	60.0%
src/likelihood/laplace.jl	4	7	57.14%
src/likelihood/generic_likelihood.jl	0	4	0.0%
src/likelihood/heteroscedastic.jl	0	4	0.0%
src/likelihood/likelihood.jl	3	7	42.86%
src/likelihood/multiclass.jl	2	6	33.33%
<!--	Total:	70	121	57.85%	-->

Files with Coverage Reduction	New Missed Lines	%
src/likelihood/laplace.jl	1	84.85%
src/likelihood/studentt.jl	1	87.5%
<!--	Total:	2		-->

Totals
Change from base Build 669:	0.9%
Covered Lines:	1449
Relevant Lines:	2537

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💛 - Coveralls