Runwhile support (limited)

[SeregaKR]

I tried to separate it into two pull requests as asked. This is the proposed change for RunWhile support.

I personally also don't have much experience with RunWhile - I'll check with some of my colleagues for use cases. For now I suggest introducing the most widely used scenarios and letting the users pass other conditions directly as string.

[SeregaKR]

Waiting for a reply from colleagues. But by quick Googling, there are a few common scenarios that I think should be implemented:

1. MaxTorque < some value
2. mx or mz (probably also my) > some value. Here it is usually passed to mumax3 as m.comp(0) for mx or m.comp(3) for mz and applying average().
   RunWhile(m.Comp(3).average() > -0.94)
3. These two cases are also often combined with a maximum time limit:
   RunWhile(m.Comp(3).average() > -0.94 && t < 5e-9)

I think we should have a parameter for the time limit, comparison (like > or <) and number. The question is should we have separate parameters for each magnetization component or just pass the name.

td.drive(system, t=5e-9, cmp=">", mz=-0.94) or td.drive(system, t=5e-9, cmp=">", p_value=-0.94, p_name="mz")