To each pump we can associate a time series defining the pump's energy
prices. This can be useful when e.g. one pumping station is diesel
powered, whereas another is electric.

Specifying energy prices is done by passing to the pumping station class
a  parameter energy_price_symbol, which can be a string or a list of
strings with the names of the corresponding time series. The
documentation has been updated with these instructions as well.

The two pumps example has been updated to show how different pump prices
can be used, and what effect they might have on the result.

Closes #58