Add ConverterApi#loss_of_load_probability

**not ready for merge yet**. @jorisberkhout needs to review this first.

... and the "demand_curve" helper methods. The latter returns the load
on the converter for each hour in the year, while the loss of load
probability returns what proportion of those hours exceed a given
production capacity limit.

  V(some_converter, loss_of_load_probability(10))
  V(some_converter, loss_of_load_probability(Q(some_query)))