Add the zoo-package to Suggestions

Author: Thomas Knecht

DESCRIPTION

@@ -55,7 +55,8 @@ Suggests:
     sf (>= 0.7-3),
     svglite (>= 1.2.0.9001),
     testthat (>= 2.1.0),
-    vdiffr (>= 0.3.0)
+    vdiffr (>= 0.3.0),
+    zoo
 Enhances: sp
 License: GPL-2 | file LICENSE
 URL: http://ggplot2.tidyverse.org, https://github.com/tidyverse/ggplot2