Handle DMOFF

[paulthebaker]

IPTA DR2 release VersionA uses DMOFF in it's .par files. My understanding is that this is similar to DMX. Neither PINT nor libstempo can handle this DM model. It has lines like:

DMMODEL DM 1
_DM  53281.0927574998 0.00145592665025232 5.36854989516535e-05
...
_CM  53341.0927574998 6.08366194548412e-07 1.10834102752821e-07
...
CONSTRAIN DMMODEL

Adding this functionality would allow us to use IPTA DR2 VersionA in addition to VersionB.

This may be a tempo2 issue and not a libstempo issue.

[paulthebaker]

duplicate of issue #15