Update beamloading

[lcarver]

Added the ability to specify an additional detuning angle in the beam loading element. When considering stability issues for active cavities, it can be desirable to apply a delta detuning angle from the optimum working point.

By specifying 'detune_angle' the feedback loop will feedback on the phase position of the optimum + detune_angle.

Below is a plot comparing the PyAT results with results based on the analytical formula (taken from MB2). There is some small disagreement, but the analytical formula contains a large amount of assumptions, whereas the numerical tracking of PyAT has fewer assumptions and also takes into account the bunch distribution

I also show here a plot showing that the detune_angle is implemented correctly, as the beam remains centered on 0 as a function of the detune_angle.

[lfarv]

@lcarver
You are experiencing a known bug in atmexall which hides the compilation errors. This has been corrected in the "crab_cavity" branch, but it's still far from being merged…
The easiest way would be to correct it here, it's such an obvious modification. Or open another PR just for that, which I would immediately approve

[lcarver]

Hello Laurent,
I have pushed the fix here. I found the bug but was trying to merge it separately. But if you think it's ok to mix the two together. I open this PR for review, which includes the beamloading update and the bugfix for atmexall