Add checks on arguments

[JoFrhwld]

There should be checks on the arguments passed to density_area() and density_polygons(), e.g.

• x, y need to be numeric
  • functions should error with helpful message
• x, y should be the same length
  • functions should error with helpful message
• x, y should have a minimum length.
  • Unsure what the length should be?
  • functions should return a 0 length tibble with a warning to safely work with tidyverse. e.g. a data frame gets grouped, and most groups have enough tokens, but one doesn’t.
• probs should be >0 and <1.
  • functions should error with helpful message