Implement wrappers for existing external mechanistic 'SDMs'

[Martin-Jung]

There are a number of more or less 'mechanistic' SDMs out there. Many of these packages still require or based on an initialization layers which are usually derived from correlative SDMs. Generally, it should be noted that the definition of what counts as 'mechanistic' is somewhat fluid, but in this context of the ibis.iSDM package we refer primarily to those functionalities that allow adding features not usually covered in correlative SDMs, e.g. demography, eco-evolutionary processes or population dynamics.

• Add a wrapper for the poems R-package
• Add a wrapper for the steps R-package.
• Add a wrapper for the RangeShiftR R-package so that ibis.iSDM outputs can passed on semingly
• Add some unit tests
• Write a vignette explaining the available options in the package (links to Beef up existing vignettes so that cover recent additions to the package #67 )