Unavailable area adjustment.

Changes to protected area calculation and update.

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.