implement bootstrapping method for metric uncertainty estimation

The bootstrapping method is based on a non-overlapping block sampling
with replacement, where the blocks are years of data. The number of
samples and the sample length (i.e the number of year blocks) are both
customisable.

The method is accessible both for deterministic and probabilistic
evaluation where a new axis is added. For now, the metrics for all the
samples are returned, but in the future, some summary statistics would
be implemented (e.g. quantiles or mean/standard deviation).

/!\ For determinist evaluation, the n-dimensional functionality became
    untenable such that the number of dimensions was fixed and
    restricted to 2D tensors.

New unit tests are included to test both the bootstrapping generator
and the numerical results obtained with the bootstrapping turned on.