Changes to protected area calculation and update.

Wood yields and C flux bug fixes.

Changes to avoid reading in all wood yield and C flux data.

Now keeping track of tiles within grid cells and time since conversion.
Refactored ForestManager.
Cleaned up some data structures.

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

Demand data for base year taken from FAO.
Demand projected using population and GDP.

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.

Added NATURAL_REGROWTH_TIME parameter.