WIP: Flexibile Loads

[DEUCE1957]

Warning: Work In Progress (WIP) so do not accept this Pull Request yet.

Attempt to extend Redispatching by enabling Environment's to have flexible loads. See "getting_started" / "12_Flexibility.ipynb" for example use. Related to issue #623 (will move onto load / generator shedding once this works).

Currently working:

1. Able to define "flexibility" actions (env.action_space({"flexibility":[("load_0_0", 0.1]}) with correct print string for BaseAction
2. Able to load flexible load attributes from "flex_loads_charac.csv"
3. "InvalidFlexibility" error if flexibility exceeds size of load
4. Observable change in load after executing flexibility action in environment
5. Integration with redispatch routine (flexibility and redispatch are solved together in one optim. problem)

Not yet working:

1. Observable change in actual/target flexibility after executing flexibility action in environment DONE
2. Automated tests (need new ones, currently manually testing with notebook)

Conventions:
Positive flexibility: Reduce consumption (less demand in the grid)
Negative flexibility: Increase consumption (more demand in the grid)
target_flex: Sum of all flexibility at a load
actual_flex: Sum of all physically feasible flexibility at a load (determined by Environment)