return squeezed xarray

Since the metrics are typically summary statistics, their size is not
very big, so that using xtensor instead of xarray as a data structure is
not as critical as for input data. In turn, using xarray allows for
metrics of different sizes to be returned without unnecessary size 1
dimensions (e.g. when only one threshold is given, or when no temporal
masking is performed). So all output metrics are now returned in their
"natural" shape (e.g. 1D for mono-component metrics, 2D for
multi-component metrics), plus any additional dimension linked to
multi-thresholds, multi-masking, etc.