Added the new tau_mag_gustless element to the forcing type, and set this array
in set_derived_forcing_fields if it is associated.  Also added new tau_mag
optional arguments to the public routines allocate_forcing_by_group() and
allocate_mech_forcing_by_group(), with similar mandatory arguments added to the
private subroutines get_forcing_groups() and get_mech_forcing_groups().  Tests
for an associated pointer were added before all calls setting the ustar, tau_mag
or ustar_gustless arrays, preparing for these ustar pointers only to be set when
the model is run in Boussinesq mode when the ustar actually make sense.

  Also use specific volume derivatives to calculate non-Boussinesq mode buoyancy
fluxes in calculateBuoyancy_Flux1d, leaving the Boussinesq buoyancy flux
calculations unchanged.

  All answers are bitwise identical in Boussinesq mode, but there is a new
element in a transparent type and new optional arguments publicly visible
subroutines, and answers will change in non-Boussinesq cases that depend on the
surface buoyancyFlux that is returned by calculateBuoyancy_Flux1d.