Feature request: Ability to sample momenta for HMC with mass matrix

[SamuelBrand1]

At the moment the HMC sampler only allows for the mass matrix of the momenta to be an (implicit) identity.

Gen.jl/src/inference/hmc.jl

Lines 1 to 3 in 8f2f4cc

	function sample_momenta(n::Int)
	Float64[random(normal, 0, 1) for _=1:n]
	end

It should be straightforward to allow diagonal and dense options here to bring this HMC sampler in line with common usage?

[ztangent]

Feel free to make a PR and add some tests! I think the best way to do this would be as an optional keyword argument to the hmc function.

[SamuelBrand1]

Feel free to make a PR and add some tests! I think the best way to do this would be as an optional keyword argument to the hmc function.

OK! @ztangent #559 is in. Hope its works.