Add step time/period calculation

[ulrikpedersen]

Add function(s) to calculate the motion step time given the acceleration, velocity and distance and higher level functions that pull the configuration data from the running system.
Investigate possibility of adding a sanity-check at the start of a scan, using the above functions to verify if a scan is likely to work when considering the laser pulse frequency.

[8ryn]

Functions to calculate the step time and pull the configuration data from the motor have been implemented in this branch: https://github.com/8ryn/tomoscan/tree/Period-calc
Sanity check needs a little more thought due to the pulse-frequency not being published as a PV.