Crg/forced draft stoves

[camiloramirezgo]

This PR implements the biomass forced draft improved cookstove inside the Biomass technology class. It does it by adding options to the current class, which will determine the behaviour of the class (i.e. natural draft or forced draft, and collected fuel or purchased):

• The self.collected_fuel parameter is used as a boolean condition to indicate if the fuel is collected or purchased.
• The self.draft_type parameter is used to indicate if the stove uses natural draft or forced draft

This PR closes #258

[camiloramirezgo]

Thank you Alicia! I added doc strings to the Biomass class.