Set nDOF for PVs

Currently the number of degrees of freedom is not set when creating the PV object in Allen and is zero by default.

The following discussion from !794 (merged) should be addressed:

• @wouter started a discussion: (+4 comments)

  Interesting ... The corresponding branches in LHCb and Rec have been merged a while ago. This particular modification in Allen adds functionality to convert the Allen PVs directly to the new PV format.