implemented a generalized get_capacity_limit method on tariff class, closing #25

rather than using a specific call to the capacity bandwidth limit, any charging strategy can now use the get_capacity_limit method, which works both for capacity rules and personal peak rules. Any future rule with a different capacity limit will have to add the required computation in that method.

Note: the charging strategies have not been tested with personal peak tariffs yet. This will be easier once issue #26 (closed) is implemented.