sprand as samples from sparse distribution

I was thinking, in the spirit of this package, maybe `rand(Normal(),SparseMatrixCSC,p,m,n) ` could be better expressed as `rand(Bernoulli(p,Normal()),SparseMatrixCSC,m,n)` where `Bernoulli(p, Normal())` would be the "Gauss-Bernoulli" or "Spike-and-Slab" mixture distribution

P(x) = (1-p) delta(x)+ p Normal(x)

It seems to make things a bit more generic.