Add kron specialisation for sparse and dense mixes.

[Ericgig]

Description
In general, operation between sparse and dense return dense. This is not ideal for the kron product which output would be sparser than the sparse input. The dispatcher does not have fine control for these cases, (Jake plan to add it but never had the time to get to it), so I added wrapper specialisation for these cases.

I had to patch expand_operator since it used dense priority to work.

[coveralls]

coverage: 86.048%. remained the same
when pulling 1a5a8d7 on Ericgig:misc.kron_dense_csr
into 8035590 on qutip:master.