cleanup raster/sf use WIP

[mdsumner]

tentatively , I'll just sit on this a while but see branch remove-raster-method

• remove raster() method
• re-export both plot() and raster() from raster
• remove sp and methods dep (because we aren't defining a raster('sf') method

need to determine sensible crs behaviour (we were copying the sf$crs$proj4string to r@crs when using raster(sf))

situations are

-[x] input sf has NA crs, do nothing
-[x] input sf has crs, input raster has NA crs ,do nothing
-[ ] input sf has crs, input raster has crs and they are different
-[x] input sf has crs, input raster has crs and they are same, do nothing

the only thing to do is st_transform when both are set but different, but do we check if there's wkt/projstring in the raster?

[mdsumner]

see https://github.com/diminutive/hackspatialcpp for some Cpp that gets what we need from sf

the removal is now merged into master with #52