compatibility module as we transition to an FMSConstants module

Constants have been declared as type REAL, PARAMETER.

The value a constant can not be changed in a users program. New constants can be defined in terms of values from the constants module using a parameter statement.

The name given to a particular constant may be changed.

Constants can be used on the right side on an assignment statement (their value can not be reassigned).

Example:

    use constants_mod, only:  TFREEZE, grav_new => GRAV
    real, parameter :: grav_inv = 1.0 / grav_new
    tempc(:,:,:) = tempk(:,:,:) - TFREEZE
    geopotential(:,:) = height(:,:) * grav_new