Make sf package optional

[pitkant]

Fix to issue #213, effectively making get_eurostat_geospatial function produce an error:

Loading required namespace: sf
Failed with error:  ‘there is no package called ‘sf’’
'sf' package is required for geospatial functionalities

Package check produces 1 note when sf is not installed:

checking package dependencies ... NOTE
Package suggested but not available for checking: ‘sf’

The cleanest solution might be to put geospatial functionalities in a new package but maybe this PR is not the place for that discussion

[antagomir]

Yes - separating geospatial functionalities has been discussed in the past as well, and could make sense. Let's keep up the planning!