Bypass flux routines if do_flux=.F.

[JosephMouallem]

Complementary to #134, add do_flux flag for routines coupler_generate_sfc_xgrid and flux_exchange_init.
Since no fluxes are being shared between components, there is no need for these routines.
With an appropriate namelist setup, full coupler shield can run with no coupled mosaic files.

[rem1776]

@JosephMouallem There are some calls to coupler_flux_init_finish_stocks in coupler main which calls the flux_init_stocks and flux_check_stocks routines from the flux exchange module. Should these get a similar update and only run if do_flux is true?

FMScoupler/full/coupler_main.F90

Lines 407 to 408 in c589a98

	if (check_stocks >= 0) call coupler_flux_init_finish_stocks(Time, Atm, Land, Ice, Ocean_state, &
	coupler_clocks, init_stocks=.True.)

FMScoupler/full/coupler_main.F90

Lines 674 to 675 in c589a98

	if( check_stocks >=0 ) call coupler_flux_init_finish_stocks(Time, Atm, Land, Ice, Ocean_state, &
	coupler_clocks, finish_stocks=.True.)

[JosephMouallem]

Hi @rem1776, they currently dont crash the model and print 0 values for the stocks in the log file, we can add do_flux to make things cleaner.

[rem1776]

@JosephMouallem Thanks for making those changes!