Avoid generating the full cpds array when calculating CRPS

[henrikbostrom]

A memory issue (#9) was fixed in version 0.5.0; instead of always generating the cpds array within the predict method of ConformalPredictiveSystem, this is now done only when requested (by return_cpds=True) or when including "CRPS" among the metrics to evaluate (which currently is the default). The latter should preferably be calculated without generating the full cpds array (which may consume a lot of memory for very large calibration and test sets).