Move barotropic time stepping into btstep_timeloop

  The barotropic solver time stepping loop was moved into the new routine
btstep_timeloop for a more modular code structure that will (hopefully) be more
likely to be fully optimized.  The time averaged barotropic pressure force and
Coriolis acceleration diagnostics were also renamed to reflect their purpose,
for example from PFu_bt_sum to PFu_avg, and separate blocks were added to write
these diagnostics depending on whether they need to be renormalized.  Also
corrected a handful of spelling errors in comments and non-standard spacing.
All answers are bitwise identical and no public interfaces were changed.

Author: Hallberg-NOAA