[Enhancement] Support a validation comparison wrapper to compare different predictions

[Martin-Jung]

Currently validate can be run on individual raster and fitted model objects, but there is no way to compare two different spatial predictions in terms of their metrics. The idea is thus to add a handy function called compare() that acts as a wrapper on validation() outputs (or directly on model objects?).
Steps for implementation:

• Ensure that each validate() output has an attribute that identifies the tibble as such.

• Implement a C3 call for compare() on >= 2 outputs of these. The compare function then calculates the differential between the best and the next models and sorts the output (default parameter for sorting provided).

• When called on a list the function should also work.

• (Optional): Evaluate whether it is possible to run compare() directly on DistributionModels instead of validation outputs. This requires that each model is of the same family and/or can return some sort of parsimony criterion (AIC, BIC, WAIC, LOO, etc...). This likely requires considerably more work...

[Martin-Jung]

Largely been superseded by a different issue ( #72 ) which should also support validation as part of the comparison.