mosaic cleanup and rename to grid_utils

[mlee03]

Description
In this PR,

1. The mosaic directory has been renamed to grid_utils.
2. The files mosaic_utils.c/h have been renamed to grid_utils.c/h
3. create_xgrid.c/h files have been moved to horiz_interp/include and have been renamed to horiz_interp_conserve_xgrid.c/h
4. The unused files - interp.c/h, mosaic.F90, grid.F90, read_mosaic.c/h- have been removed. The sole function that is used in read_mosaic - get_global_area has been moved to grid_utils.c/h
5. Functions used for working with the tree data structure in the great_circle algorithms have been moved from create_xgrid.c/h and grids_utils.c/h to the new files called tree_utils.c/h
6. All non-create_xgrid methods in create_xgrid.c/h have been moved to grid_utils.c/h
7. Unused functions in grid_utils.c/h haave been removed. These include box_area, nearest_index, tokenize, grid_box_radius, dist_between_boxes, box_ctrlat, box_ctrlon, and get_grid_area_dimen
8. A simple unit test ensuring that create_xgrid_2dx2d_order2 can be invoked successful has been added. (The currently existing horiz_interp unit tests already test for create_xgrid_2dx2d_order1)

With these changes, grid_utils now consists of

• constants.h
• grid_utils.c/h
• tree_utils.c/h
• gradient_c2l.c/h (functions are used by gradient_mod)
• gradient.F90 (the module is used in exchange_mod)

Grid_utils is now used by exchange, mosaic2, and horiz_interp

Horiz_interp/include now in addition holds horiz_interp_conserve_xgrid.c/h .

Fixes #1618

How Has This Been Tested?
FMS compiles with cmake and autotools. Horiz_interp unit tests pass.

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

[rem1776]

Changes look good, just had two small comments before i approve.