Warn when imported basedata has only one cell

[shawnlaffan]

A relatively frequent issue is when users import data in geographic coordinates, but use the default cell size of 100,000.

Some documentation is in the FAQ, but who looks for such things these days?
https://github.com/shawnlaffan/biodiverse/wiki/FAQ#i-imported-my-data-and-it-only-has-one-cell

It would be useful if Biodiverse warned users when this is the case.

We could try to detect an appropriate cell size from the first few rows of data, but that is prone to errors and issues such as low values in the first set of rows.