Stochastic solvers small bugfix

[pmenczel]

When calling ssesolve or smesolve without any stochastic collapse operators but store_measurement set to True, there is an IndexError. This PR fixes the error and adds a test for this situation.

Example code:

import qutip as qt
r = qt.ssesolve(qt.sigmaz(), qt.basis(2, 1), [0, 1, 2], options={'store_measurement': True}, ntraj=15)

[coveralls]

coverage: 86.094%. first build
when pulling 9e628b3 on pmenczel:stochastic-fixes
into d8c6584 on qutip:master.

[Ericgig]

Nice catch.

Without sc_ops the stochastic solver is simply me/sesolve, maybe we should also add a fallback like mcsolve.