BField caching and handling Updates
This closes ACTS-385 and closes ACTS-379.
It introduces the caching of the BFieldCell (not only the BField value), and allows for interpolation for each bField call as long as one stays within the cell.
It also introduces (for the Propagation to a target Surface only) a call pattern where the caller, who might want to do consecutive field calls, such as e.g. during Extrapolation and or Kalman Filtering, can provide and update and external cache.
Edited by Andreas Salzburger