made tariff rules explicit at instantiation of a tariff and in accessing...

made tariff rules explicit at instantiation of a tariff and in accessing tariff attributes, closing #24 (closed)

the tariff class now has both a list of rules and each rule also explicitly as an attribute. Perhaps this is a bit redundant and could be done differently. For now I kept it because the list is used in computing the total charge, while the individual tariff rules can be easier to use when accessing a specific parameter like tariff.capacity_rule.capacity_bw