2624 bugfix mov count

This merge request closes 2624 and adds the following features:

• mov_count and rel_mov_count plan will now reset the velocity to whatever it was before the scan was started, even if the plan is aborted

• rel_mov_count is created. It's the same as mov_count, but moves a set distance from the initial position.

• If an exception is called from the motors, detectors, or ctrl+c then the velocity is set back to the original value. in both rel_mov_count and mov_count the initial position is not set again if the plan is aborted. If the mov_count plan is called again after RE.abort() then the initial start position will be reset. If rel_mov_count is called again it will move from where ever it stopped at.

• Pytests check that the velocity is reset in both cases and that the functionality described is achieved

If it was required that rel_mov_count should move back to the start in an exception, we could add another exception case for it which offered the chance to move back to the start if you wanted. This is not implemented in this merge request.