PINT should have a TCB to TDB conversion script

[abhisrkckl]

Right now the only way to convert from TCB to TDB is to use the TEMPO2 transform plugin. This feature will allow PINT users to deal with TCB par files even if they don't have TEMPO2 installed.

See also: Issue #37, Issue #1403, PR #465, PR #1404

[abhisrkckl]

The transform plugin does the following:

For mjd parameters, it does
t_tdb = (t_tcb - tref)*f + tref

For other parameters with dimensions [T^n L^m ...], it does
A_tdb = A_tcb * (f**(m+n))

Exceptions are DM and its derivatives, and parallax (and DMX/DMMODEL parameters, I guess). They enter the timing model multiplied by the DM constant and AU. Hence, they have different "effective dimensions".

[abhisrkckl]

Interestingly, the transform plugin does not scale the DMX parameters. This probably explains why I was getting ~10 ns errors for InPTA data with TDB par files generated using the transform plugin.