VAE allows only CGaussian

[smidl]

Can we allow VAE for AbrstactPDF and AbstractCPDF? As it is now, I can not use SharedVarCGaussian in VAE.

[nmheim]

Would it still be a VAE if it was an AbstractCPDF? We also have:

abstract type AbstractCPDF{T<:Real} end
abstract type AbstractCGaussian{T} <: AbstractCPDF{T} end
struct CGaussian{T,V<:AbstractVar} <: AbstractCGaussian{T} end
# same for SharedVarCGaussian

so we could also make it a AbstractCGaussian. I can change it to either one.

[nmheim]

closed with #36