Add option to limit DCP output to the "Bv2.0 profile" (#2470).

Add option to limit DCP output to the "Bv2.0 profile" (#2470).

I'm far from convinced about the point/sense of all these "profiles"
(rather than just implementing or at least tolerating the standard)
but lots of people are having problems with "QC" processes failing
their DCPs with complaints related to MCASubDescriptors.  It seems to
make sense to have an option to turn them off - at least for now,
until either the "QC" situation settles down or any bugs in DCP-o-matic
are found and fixed.