fregrid_acc: change constants to "const data_type var_name = value"

[mlee03]

Is your feature request related to a problem? Please describe.
In current libfrencutils, constants in constants.h are defined as macros and are handled by the preprocessor. However this is not the preferred method for fregrid_acc implementation as it could lead to bugs, maintainability issues, and loss of precision.

Describe the solution you'd like
Constant values in libfrencutils/constants.h will be defined as const data_type var_name = value in a new file called libfrencutils_acc/constants_acc.h. All fregrid_acc related code will #include constants_acc.h

Describe alternatives you've considered
Leave as is

Additional context
N/A