Replace wellknown with wk

[salvafern]

There is a dependency on wellknown.
https://github.com/ropensci/mregions/blob/master/R/as_wkt.R#L44

This package will probably be archived soon due to a problem on CRAN: ropensci-archive/wellknown#36

We have to replace wellknown with something else. Possible candidates: wk or sf.

[salvafern]

I switched the mr_as_wkt.mr_geojson method to a combination of functions from geojson, geojsonio and geojsonsf. The output may not be exactly the same than the previous version. We should consider to increase the package version. In any case this function will be deprecated after the main update in #62