how to use optimal_capacity()

[Matteo De Felice]

Good evening,

is it possible to get the optimal_capacity() for each bus? The documentation is a bit obscure, unclear to me how to use `groups`, `aggregate_groups`, etc.

Also, the example page is broken now: Using the statistics module in PyPSA — PyPSA: Python for Power System Analysis

Has anyone some interesting examples to share? I have the impression to use only a fraction of the `statistics` module and I would like to use it more! 

Best regards,

Matteo