move {mgcv} from Imports to Suggests

Author: teunbrand

DESCRIPTION

@@ -40,7 +40,6 @@ Imports:
     isoband,
     lifecycle (> 1.0.1),
     MASS,
-    mgcv,
     rlang (>= 1.1.0),
     scales (>= 1.3.0),
     stats,
@@ -56,6 +55,7 @@ Suggests:
     knitr,
     mapproj,
     maps,
+    mgcv,
     multcomp,
     munsell,
     nlme,