Fix sink potential check

[conradtchan]

Type of PR:
Bug fix

Description:
Required for #217, discovered when mpirun -n 4 was added

In ptmass.F90, during sink creation, whether the particle is at the potential minimum is checked:

       ! CHECK 7: particle i is at minimum in potential
       if (poteni > potenj_min) then
          ifail = inosink_poten
          ifail_array(inosink_poten) = 1
       endif

This check fails because an identical particle distribution gets created on every task, resulting in close comparisons when determining which particle to create a sink at.

This is fixed by only creating the particles on the master task, and allowing derivs to rebalance them.

itest needs to be calculated after calling derivs or else it will be incorrect. The xcofm calculation has been removed because the result is not used anywhere.

Testing:

make MPI=yes SETUP=testgrav phantomtest
mpirun -n 4 bin/phantomtest ptmass

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[conradtchan]

Note that the automated test suite does not cover the bug that this PR fixes because #217 has not been merged yet. It needs to be tested by running the commands given in the description.