Fix #1691

[AGaliciaMartinez]

Description
Changes pow to allow the exponentiation of arbitrary square matrices regardless of their type as qobj.

I tried to change it so that bra*ket returns a scalar but this breaks backwards compatibility and there are modules such as control that do thinks like

(bra*ket).tr()

or

(bra*ket).data[0,0]

that would stop working.

Related issues or PRs
fix #1691

Changelog
It is now possible to compute the power of an scalar-like qobj.

[hodgestar]

This looks like a good minimal change to me.

I've flagged it for inclusion in QuTiP 4.6.3.

We've had some odd test failures recently that haven't been tracked down yet. I would like to only merge this once all tests pass, but let's see how the test runs go.

[hodgestar]

@AGaliciaMartinez Would you mind updating this branch to the latest master branch? (I am trying to understand the broken test)

[AGaliciaMartinez]

@hodgestar Should be now updated.

[coveralls]

Coverage increased (+0.06%) to 67.127% when pulling 59e4ddd on AGaliciaMartinez:fix#1691_allow_pow_with_scalars into 981328c on qutip:master.

[hodgestar]

Since tests pass with the updated branch, I'm going to merge this.