Tests fail if {rsgeo} is not installed

[Robinlovelace]

From CRAN checks:

  > # rsgeo implementation (default if available):
  > if (rlang::is_installed("rsgeo")) {
  +   rsmulti = line_segment(l, segment_length = 1000, use_rsgeo = TRUE)
  +   plot(rsmulti["ID"])
  + }
  > # Check they have the same total length, to nearest mm:
  > # round(sum(st_length(l_seg_multi)), 3) == round(sum(st_length(rsmulti)), 3)
  > # With n_segments for 1 line:
  > l_seg_multi_n <- line_segment(l[1, ], n_segments = 3, use_rsgeo = FALSE)
  > l_seg_multi_n <- line_segment(l$geometry[1], n_segments = 3, use_rsgeo = FALSE)
  > l_seg_multi_n <- line_segment(l$geometry[1], n_segments = 3, use_rsgeo = TRUE)
  Error in loadNamespace(x) : there is no package called ‘rsgeo’

Source: https://cran.r-project.org/web/checks/check_results_stplanr.html

cc @JosiahParry I plan to work on this and put in a PR for you to review. If you have any quickfire suggestions let me know.