Add support dense matrix representation in Qobj

[jrjohansson]

Qobj currently use sparse matrix representation for operators and kets/bras, which is optimal for large systen but suboptimal for small systems. We want Qobj to support both sparse and dense matrix representation for the quantum object it represents. This should probably be implemented with two different classes, QobjSparse and QobjDense which inherit a common interface from a parent class Qobj.

[nonhermitian]

I doubt we will ever get around to this, so I will close this issue.