biogeochem basic functionality working

The basic functionallity for the biogeochem module is working. Tests
with several different elements, species, substrates and biomass are
working.

Several notes:
1) There's no error checking if the name of one component is written
wrong somewhere. The concentration will be given the value zero
and it will be carried around.

2) The number of components still needs to be checked. All new
components (like solid solutions) that need to be added needs
to be checked and code added in _both_ routines.

3) The value itself that is given back from each routine must be
checked.