Require always sf for get_eurostat_geospatial()

[dieghernan]

Close #279

@pitkant maybe you want to have a look first

[pitkant]

Very good PR as always, thanks! I ran tests on this version with and without giscoR and sf packages and things went smoothly. I will have to check CRAN guidelines whether or not it's a problem that there's inheritDotParams from giscoR that I think causes the Rd xrefs note here:

❯ checking package dependencies ... NOTE
  Packages suggested but not available for checking: 'giscoR', 'sf'

❯ checking Rd cross-references ... NOTE
  Package unavailable to check Rd xrefs: ‘giscoR’

0 errors ✔ | 0 warnings ✔ | 2 notes ✖

R CMD check succeeded