ADDED: check for dims in expect

Switch to cython calls for expect gave cryptic error message if dims of
objects were not aligned.
ADDED: steady_state_average flag

When using time averaging in place of ensemble averaging for steady
state calculations using mcsolve, setting this flag to True does the
averaging internally over the supplied timesteps and returns a single
Qobj that is the time-averaged density matrix provided that the system
is in a steady (stationary) state.