Generation mixes for pumped hydro

@bkuczenski @cdgaete @ntropy-esa @jakobsarthur

We need a strategy on how to account for the electricity used to charge pumped hydro (we don't have any other storage in the API, at least as far as I can tell).

The problem is that pumped hydro is a generator, but they don't list it as a load. We need to keep it in the generation mix to have energetic balances.

The API does have [filling of hydro storage plants](https://transparency.entsoe.eu/content/static_content/Static%20content/web%20api/Guide.html#_generation_domain) (search for "Aggregated Filling Rate of Water Reservoirs and Hydro Storage Plants") - but this is only weekly totals.

ecoinvent and [electricitymap.org](https://github.com/tmrowco/electricitymap-contrib/blob/master/config/co2eq_parameters.json#L34) just use the average generating mix - which itself already includes pumped hydro ([gasp!](https://www.youtube.com/watch?v=cphNpqKpKc4)). I also asked the [open energy modelling folk](https://forum.openmod-initiative.org/t/modelling-grid-mix-used-to-charge-pumped-hydro-other-energy-storage/1464), but didn't find anything helpful.

So, the best I can some up with is to charge with the weighted average mix when pumped hydro is not being used, with weights coming from the inverse of the electricity price - the assumption being that lower prices are more likely to times when pumped hydro would be charged. But maybe you have a better idea? You can see the available data in the [example notebook](https://github.com/BONSAMURAIS/bentso/blob/master/Basic-API-usage.ipynb).