Changes

I have passed clubb_params down into advance_xm_wpxp_module.F90.

The results from this revision are an exact match to the results from
the previous revision for all cases.

I have passed down CLUBB's tunable parameters into advance_wp2_wp3_module.F90.

The results in NetCDF output files from this revision are an exact match to the
results in NetCDF output files from the previous revision for all cases.

I passed down CLUBB's tunable parameters into advance_xp2_xpyp_module.F90
for most parameters, excluding C4 and C14. Results are unchanged.

Passing down more of CLUBB's tunable parameters.

I accidentally removed an important comment in the previous commit.

Passing down more CLUBB tunable parameters.

Everything is bit-for-bit.

More passing down CLUBB parameters ... whoop-de-doo

Passing down more tunable parameters and removing references to
parameters_tunable. Results are bit-for-bit.

Handling the abomination that is lmin.

Passing tunable parameters into the new tau code.

Passed down more parameters. Results are bit-for-bit.

Passing through more clubb parameters. Bit-for-bit

Passing down CLUBB's tunable parameters into the setup_pdf_parameters and
precipitation fraction portions of the code. Everything is bit-for-bit.

I passed down CLUBB's tunable parameters into the advance_microphys portion of
the code. It's all bit-for-bit.

Passing down CLUBB's tunable parameters into the remainder of the
PDF subroutines. Everything is bit-for-bit.

I passed down the forgotten thlp2_rad_coef tunable parameter.
All bit-for-bit.

Passing down CLUBB's tunable parameters into CLUBB's
microphysics initialization.

I am passing down the forgotten parameter, altitude threshold, in
advance_xm_wpxp_module.F90.

I am committing the final stage of the "pass-down params" project.
The results are bit-for-bit identical.

Adding an option to use invrs_tau_N2_iso for C4 terms.  The same invrs_tau had been used for C4 and C14 terms, so some adjustments had to be made to separate them out (this was started with an earlier commit: https://github.com/larson-group/clubb/commit/ef3be1b1378bcd1a447f3866c9f88f8cc4764233).  The flag is currently set to false so results will be b4b.

See also the discussion here: https://github.com/larson-group/clubb/issues/920#issuecomment-920338746 and following comments.

Renaming invrs_tau_zt and invrs_tauw3t to "invrs_tau_wp3_zt" in advance_wp2_wp3 to avoid confusion.  This variable is only used for the wp3 C8 term, so one consistent name is simpler, and the name "invrs_tau_zt" is already used by another variable in advance_clubb_core.

Adding comments for clarity.