Closes #912 #1102
addresses #1106

Changes proposed in this Pull Request

Introduce a new plotting accessor scheme, to quickly generate plots based on the statistics module (as described in #1106). The implementation now only works for static plots. Run it with n.plot.map.energy_balance or n.plot.bar.optimal_capacity etc.

Future work

• think about moving code to components class where possible to avoid redundancy
• extend for multiinvestment networks (have stochastic networks in mind)
• extend for interactive plots, via iplot. we can recycle a lot of code here
• extend for plotting multiple networks at once

Checklist

• make the old namespace pypsa.plot deprecated in favour of pypsa.plot.maps
• autogenerate_coordinates was renamed to apply_layouter (keep backwards compat)
• @lkstrp I kept the namespace order now this way, as it helped me structuring things, I think we can still change if to n.plot.energy_balance.map, once we agreed on the fundamentals.
• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.