Various fixes.
Code cleaned up.

Created forestry package.
Added readers for forest growth data.
Added ForestManager.
GAMS optimisation rewritten in terms of Net Present Value.

Max. wood from natural areas harvested at replacement rate.
Effect of previous land cover on wood yield only lasts for one timestep.
No wood harvest from agricultural expansion.
Added variable tree logging cost (fixes oscillating export issue).
Timber and carbon production now serialized during calibration.