Pre-set beamshift before FastADT to avoid missing first frame

[Baharis]

In the current version of FastADT experiment, the loop responsible for collecting the data typically looks something like this (simplification):

movie = self.ctrl.get_movie(n_frames=len(run) - 1, exposure=run.exposure)
with self.ctrl.stage.rotation_speed(speed=rot_speed):
    self.ctrl.stage.set(a=target_alpha, wait=False)
    for step, (image, meta) in zip(run.steps, movie):
        # this code runs only after movie image is collected
        if run.has_beamshifts:
            self.ctrl.beamshift.set(step.beamshift_x, step.beamshift_y)

The beamshifts, collected and extrapolated before diffraction experiment, are applied only after the stage movement and movie collection has already started. This means that the first collected image is never preceded by any beamshift.set statement. Unless the beam randomly happens to be at a correct position, the first frame will be always empty and should be rejected.

This PR adds a new beamshift.set command just before opening the beam blank, which itself introduces small delay, so that the first image already has a beam deflected towards the starting position. Since the beam shifts can now be taken from a file following #159, it also checks if none of the beam shifts is NaN.