Refactor diapyc_energy_req_calc and find_PE_chg

  Modified the MOM_diapyc_energy_req.F90 version of find_PE_chg to align more
closely with the version in MOM_energetic_PBL.F90, including making PE_chg into
a mandatory argument, changing the name of the ColHt_cor argument to
PE_ColHt_cor, and modifying some variable descriptions in units.  Also removed
find_PE_chg_orig from MOM_diapyc_energy_req.F90 and the old_PE_calc code that
calls it.  Extra values were also added to Te, Te_a and Te_b and the equivalent
salinity variables so that the logical branches at (K==2) and (K=nz) could be
simplied out of diapyc_energy_req_calc.  Because old_PE_calc had been hard-coded
to .false., all answers are bitwise identical.