deprecate use of sp package

[adokter]

• package sp will be superceded by sf for vector format spatial data.
• Package raster remains default choice for raster data

Suggestions Matt Strimas-Mackey:

sp is quickly becoming obsolete. SpatialGridDataFrame isn't a widely used format anyway. Raster is the way to go for gridded data. One huge benefit is that it doesn't read the data into memory like most data in R, instead it pulls it in chunks from the file. Gridded data can become quite large, so this avoids using up all your memory.

Two other options to consider:

• https://github.com/r-spatial/stars is a new package specifically for dealing with spatiotemporal data. It's not widely adopted yet, but it seems like it could be perfect for our use case.
• https://github.com/rspatial/terra this package will apparently replace raster. I think it's a long way from being done, so probably not worth looking into now, but keep eye on it for the future.

[bart1]

A quick addition, if this is going to be implemented it might be worth to also think about integrating the use of units, both stars and sf work nicely with them. I think it is quite valuable to think about these and it is already nicely documented in the package so it should not be too hard.

[bart1]

This might relate to #255 as well

[adokter]

see also #505