pass multi-sites/multi-leadtimes tensors to probabilist Evaluator

Until now, the probabilist Evaluator was not able to compute multi-sites
and/or multi-leadtimes metrics, because it was given one site and one
lead time at a time. This was done to spare some memory, but it was not
logical to load all sites and all lead times for the input data and then
only process a small chunk at a time.

There is no multi-sites/multi-leadtimes metrics implemented yet, but
the Evaluator is ready for it now. This implementation makes full use
of the broadcasting power of xtensor.