ENH: enable writing volumetric B-Splines control points in binary

as a step towards machine-accuracy continuation of the optimisation
loop.

Additionally, control points are now written under the time/uniform
folder, to be in-line with rest of the code structure for continuation.
As a side-effect, the controlPointsDefinition in
constant/dynamicMeshDict does not need to be changed to 'fromFile'
anymore in order to perform the continuation. The 'fromFile' option is
still valid if the user wants to supply the control points manually but,
as with all other controlPointsDefinitions, it will be disregarded if the
proper file exists under the time/uniform/volumetricBSplines folder.