Add the possibility to define the data type of initial_state at simulation

[a-corni]

As mentioned in #783, the initial state is stored as a qutip object with data type CSR. This is not an issue if the state is not defined, because the default state is sparse.

However, some users might want to define a dense initial state. In this case, it would be better to store it/at least use it in a dense format in the simulation.