Improve handling of MLC and model info for DICOM

This commit introduces some enhancements for the Plan#to_dcm method:

Added logic for deciding whether the machine the plan is based on
actually has an X jaw (previously and X jaw was always inserted, which
may give an error on jawless machines such as the Siemens).

Added options for specifying some machine information which is not
possible to determine from the RTP file:
-Manufacturer
-Manufacturer's Model Name
-Device Serial Number