Storage/StoragePolicy improvements

[kilianp14]

• Completely refactored StoragePolicy to MicrogridPolicy: Microgrid can now switch between grid-connected and islanded mode in the DefaultMicrogridPolicy; apply function now returns an energy_delta instead of a power_delta; Microgrid now always has such a policy; grid_power has been changed to charge_power (meaning the power that the battery is (dis)charged with); better documentation (still not all too great Improve documentation of storage module #117); policy is now its own module; better distinction between Storage and Policy (closes Simplification of Storage/ StoragePolicy #169)
• Improved handling of storage: Update function now returns the total energy that was (dis)charged instead of a power_delta that is difficult to understand; Storage state is now monitored correctly (closes Incorrect monitoring of storage state #187) and monitor can now be initialized with policy and storage component
• Improved cosim: Grid is now stepped at the end, avoiding storage (dis)charging at the first time step; seperate aggregator now computes the p_delta at every step of the microgrid and sends it to controllers and grid; energy delta that is computed by the policy is given to the controllers in the following timestep
• updated tests and examples