Allow the use of RTPS without the need to compute posterior forward operators

[mgharamti]

Use case

Use RTPS as posterior inflation without computing posterior forward operators. At the moment, this is not possible. Note that unlike other posterior inflation algorithms in DART, RTPS is not Bayesian and doesn't require forward operator computations.

Is your feature request related to a problem?

Setting posterior inf_flavor to 4 with compute_posterior=.false. generates an error as shown in the adaptive_inflate_mod snippet below:

DART/assimilation_code/modules/assimilation/adaptive_inflate_mod.f90

Lines 470 to 475 in 146c853

	! Cannot select posterior options if not computing posterior
	if(.not. compute_posterior .and. inf_flavor(POSTERIOR_INF) /= NO_INFLATION) then
	write(string1, *) 'cannot enable posterior inflation if not computing posterior values'
	call error_handler(E_ERR,'validate_inflate_options', string1, source, &
	text2='"compute_posterior" is false; posterior inflation flavor must be 0')
	endif

Describe your preferred solution

I think we can modify the above if statement to exclude RELAXATION_TO_PRIOR_SPREAD from the check.

Describe any alternatives you have considered

N/A