Add conservative support in time_interp_external

[mlee03]

Description
This PR adds support for conservative horizontal interpolation in time_interp_external_mod. Users can now pass in the horiz_interp_type interp with interp%interp_method = CONSERVE in order to remap the external data to the model grid before time interpolation.

Code change explanation:
Horiz_interp_conserve methods do not accept mask arrays for data interpolation. (Users are expected to pass in the mask arrays during weight generation). Thus, (1) mask arguments were removed in call horiz_interp in the load_record subroutine and (2) a warning message has been added to warn users to check if the interp derived type correspond to the interpolation region.

Fixes #1770

How Has This Been Tested?
Make check passes with GCC on the AMDbox

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes

[mlee03]

It's a draft. The new unit test doesn't pass yet.

[mlee03]

@uramirez, ready for review!