4 - fregrid_acc: cleanup and funcion get_cell_minmaxavg_latlons_acc

[mlee03]

In this PR,

1. Files fregrid_utils_acc.c/h have been changed to interp_acc.c/h. These files will contain all miscellaneous functions used in conserve_interp.c and bilinear_interp.c.
2. The function names in the directories fregrid_acc and libfrencutils_acc have been changed to end with prefix _acc.
3. File parameters.h has been created and all macros used in fregrid_acc are/will be defined in this file.
4. A new struct called Cell has been introduced. This struct will be used to store grid cell information such as the cell vertices and will be used in create_xgrid functions.
5. All non-create_xgrid functions have been removed from create_xgrid_acc.c These functions have either been removed or relocated to create_xgrid_utils_acc.c/h and general_utils_acc.c/h.
6. Files general_utils_acc.c/h have been created. These files duplicate functions in libfrenctutils/mosaic_utils.c/h. These functions will be cleaned up and if possible, optimized, to make them actually user-friendly and understandable.
7. New functions called get_cell_minmaxavg_latlons_acc and get_cell_vertices_acc have been created. These functions are used to populate the Cell struct. Functional unit test for get_cell_minmaxavg_latlons_acc has been added.

[mlee03]

closing, a larger PR will contain this PR.