Ensure dtype in Qobj creation functions.

[Ericgig]

Description
In #2314, it was found that Qobj creation functions could return an unexpected dtype for incomplete data layers.
For example if mul specialization is missing scale * basis(..., dtype) would not return a Qobj using dtype.
I reviewed functions in states.py, operators.py and random_object.py.

[coveralls]

coverage: 85.808% (+0.007%) from 85.801%
when pulling 1316bd6 on Ericgig:misc.create_to_fix
into fcb4117 on qutip:master.

[hodgestar]

We should also document the default dtypes, but this looks like a good improvement.