Library of techs

[sjpfenninger]

Problem description

Having a library of technologies (e.g., solar_pv, groundsource_heatpump) would dramatically increase the ease with which a new model can be built.

Steps to introduce the functionality

• Gather techs from existing models
• Decide on structure for storing this information and how you retrieve it
• Implement things

[sjpfenninger]

Discussion in #77 can be mined for examples which a tech library should help clarify when defining a PV tech