Add a namelist warning when hillslope hydrology is used with initial interpolation.

[mvdebolskiy]

Description of changes

Added warning to CLMBuildNamelist.pm and a warning test in build-namelist_test.pl.

Specific notes

The warning is added after logic for initial conditions is done.

Contributors other than yourself, if any:

CTSM Issues Fixed (include github issue #):

Fixes #1806

Are answers expected to change (and if so in what way)?

No.

Any User Interface Changes (namelist or namelist defaults changes)?

You have to run ./xmlchange --append CLM_BLDNML_OPTS=-ignore_warnings if you want to have use_init_interp with use_hillslope both true.

Does this create a need to change or add documentation? Did you do so?

Testing performed, if any:

ran build-namelist_test.pl and tested a new case with above namelist changes

[samsrabin]

Converting to draft, pending the rebase.