Ability for Free Colonists to gain experience indoors

[HighwayofLife]

The current default behavior of FreeCol is that outdoor activities all carry up to a 4% chance that a Free Colonist could become an expert in the production area that they are working, regardless of skill level, all outdoor skills have up to a 4% chance, but there currently is no chance that a Free Colonist can gain experience when working indoors.

This Change:

• Adds the ability for Free Colonists to gain experience when producing goods indoors and become experts.
• Percentage (probability) is based on the skill level of the position.
  • The Carpenter is skill level 1, so has up to a 4% chance
  • Statesmen and Preachers are skill level 3, so have up to a 2% chance
  • All other occupations are skill level 2 and thus carry up to a 3% chance

[Blakeley00]

This is a really good idea too!

[HighwayofLife]

Recreated in PR-135: https://github.com/FreeCol/freecol/pull/135