Helper function to compare two or more predictions in terms of value and configuration overlap

[Martin-Jung]

A common issue with SDM projections is not only the validation of them, but also the comparison between predictions. In other words how similar in values, overlaps and even spatial configuration are two or more SDM projections?
We could think of adding a helper function to the package that allows to assess this.

• Implement a wrapper method (compare() ?) that compares two or more predictions in terms of their overlap
• Basic functionality could be a simple value comparison for continuous (e.g., pearson r, bray-curtis dissimilarity) or categorical data (e.g. sorensen similarity index, Schoner's D see Broenniemann et al. 2011).
• Or similarity in terms of spatial configuration such as enabled via the motif package ?
• Also support validation by providing a common metric and individually call and then rank (validate())
• Add some unit tests and test data.
• Small vignette entry

[Martin-Jung]

@mhesselbarth See also issue for ideas in supporting Niche overlap functionalities.
Feel free to align your commits with that one.