In this section, different utility modules and routines are assembled,
such as the `time` module (see `time.F90`), keeping track of all time
calculations, the `mtridiagonal` module with a Gaussian solver for
systems of equations with tri-diagonal matrices (see `tridiagonal.F90`), and the `eqstate` module (see `eqstate.F90`) with different versions of the equation of state.

Also discussed are advection and diffusion routines, such as `diff_center()` and `adv_center()` for variables located
at the centers of the grid cells, i.e. in general mean flow
variables.

Karsten Bolding 2012-01-24