Update stability functions

INTERFACE:

   subroutine stabilityfunctions(nlev)

DESCRIPTION:

Based on the user's specifications in gotmtub.nml, this internal routine selects the desired stability functions defined in (44). These simple functions depend on $\alpha_M$ and $\alpha_N$ defined in (45), which are in most cases only used to compute the Richardson-number

$$Ri = \dfrac{\alpha_N}{\alpha_M} \quad .$$ (119)

A description of individual stability functions starts from section 4.28.

USES:

   IMPLICIT NONE

INPUT PARAMETERS:

   integer, intent(in)                 :: nlev

REVISION HISTORY:

   Original author(s): Hans Burchard, Karsten Bollding, Lars Umlauf