Make set_operation method public

[misi9170]

As mentioned in #865, there are some cases where users may want to set only the control setpoints. Currently, the method for doing this, FlorisModel._set_operation(), is private. This PR makes that method public so that it may be used.

We intend that only power users will use this functionality---most will simply use the set method to alter the control setpoints.

In making this change I found that there was no check that the yaw_angles or power_setpoints second dimension matched the number of turbines, and in fact it was possible to set and run yaw_angles with more than the number of turbines. This is now checked for.

I spoke to @rafmudaf about a possible issue created by the following workflow:

fmodel.set_operation(yaw_angles=some_yaw_angles)
fmodel.run()
fmodel.set_operation(yaw_angles=some_other_yaw_angles)
powers = fmodel.get_turbine_powers()

This would be bad, because the power would have the wake caused by some_yaw_angles but the power behavior caused by some_other_yaw_angles.

To make sure this does not happen, I have set the core's state back to UNINITIALIZED if set_operation() is called. This then throws a RuntimeError if get_turbine_powers() is called before calling run() again. I test for this in the tests.

[paulf81]

This looks solid @misi9170 thanks!

[rafmudaf]

Nicely done. Just a heads up that the State attributes might not be rigorously used or checked. It looks like it works great in this case, but it might not generally work in the future. Or it might, I'm just not entirely confident about it.

[misi9170]

I'm going to wait to merge until I get response on #865---that should be fine though, this doesn't really affect any other open PRs.

[misi9170]

Thanks @achenry for calling this out!