move fms2io_interpolator_init to interpolator_init

[mlee03]

Is your feature request related to a problem? Please describe.
In interpolator_mod, the subroutine interpolator_init mainly calls the subroutine fms2io_interpolator_init. The code under fms2io_interpolator_init does not need to be in a separate subroutine. All initialization code can be moved to interpolator_init

Describe the solution you'd like
Combine interpolator_init and fms2io_interpolator_init; and remove fms2io_interpolator_init.

Describe alternatives you've considered
Leave it the way it is.

Additional context
Removing fms2io_interpolator_init will result in a cleaner code. Else, the users will wonder why there is a separate subroutine fms2io_interpolator_init.