Added NATURAL_REGROWTH_TIME parameter.

Further forest work on GAMS code.

Fix for land allocation error in allocAreas method. Unprotected natural area was being calculated on more that one occasion within a timestep resulting in changing proportions of 'other natural' and 'forest' that should be protected. Area(s) of natural land that should be protected now updated at the start of each timestep and remain constant throughout the timestep.