biogeochem module created, first subroutine included

The module biogeochem was created and the first subroutine,
print_components, is working. Some things must be kept in mind
when interfacing with iphreeqc:

1. include iphreeqc.f90 must NOT be used in the module declaration
as it will conflict with the iphreeqc module.

2. The kind=8 for real variables also must be used to avoid
conflicts with iphreeqc. Not really useful, but without it
the compiler throws an error of wrong interface type.

3. If id is created outside the module it can be used inside it,
but not changed because of the intent(in). All other variables
can be changed from outside.