sealevel_nandum. Helper routine to spot NaNs

INTERFACE:

    subroutine sealevel_nandum(a,b,idum)

USES:

INPUT PARAMETERS:

    REALTYPE, intent(in)       :: a,b

OUTPUT PARAMETERS:

    integer, intent(out)      :: idum

DESCRIPTION:

This routine is a kind of dummy routine primarily to provide a means to spot NaN values. Output is 1 or 2, based on which is smaller (a or b, respectively). The default is 2, and the idea is that "imin=2" should be returned also if a is NaN. If b=HUGE(b), then this provides a means to detect if a is a denormal number.