Migrate ppi data to terra

[iskandari]

• migrates ppi$data from SpatialGridDataFrame to terra::SpatRaster across PPI creation and downstream methods
• adds unit tests

[bart1]

@adokter / @iskandari Quick idea would it be an option to use the terra object directly/using class inheritance. That would make all terra/spatial functions work directly on the results of a project_as_ppi function instead of always having to address a slot in the ppi object