improved support/documentation for BGC variable access from the host

[jornbr]

Based on user feedback:

Several users would like to see improvements that make it easier and more flexible to get hold of specific biogeochemical variables for use within the physics model.

In tandem, documentation on this topic could be improved/expanded: it is not very clear how to get/couple dependency from a physical model – especially if the dependency is not on FABM’s list of standard variables.