Add base electricity demand profile

[brynpickering]

Base electricity demand will be added to with heat and EV demand (see #67 & #68). It is its own separate load with separate DSM potential. To generate this profile, we will use the 2012 demand profile and scale according to the regional annual FES demands, including some DSM potential as also given by the FES.

• Process regional annual demand from FES sheet BB1. Use "Baseline Demand" as it does not include EV/HP demand.
• Create 2012 electricity load profiles using existing PyPSA-Eur functionality.
• Scale annual profile according to annual values.
• Process DSM potential from FES sheet FLX1 (Residential Peak Shifting (Smart/TOUTs effect) for residential, I&C DSR impact at peak (processes) for I&C).
• Add demand as load in compose_network script with DSM potential included.