Add warning when bandwidth < cell size

[mpjashby]

Is your feature request related to a problem? Please describe.
When the bandwidth approaches the cell size, the KDE estimates will fall to zero because no cells will be included in the density estimation.

Describe the solution you'd like
A warning if the bandwidth is either less than the cell size or less than whatever multiple of the cell size means that no cells are included.

Describe alternatives you've considered
The warning could be an error, but a warning is probably best because there may be unforeseen circumstances in which a small bandwidth is desirable.

[mpjashby]

This is probably best dealt with in the validate_bandwidth() function:

sfhotspot/R/validate_inputs.R

Lines 130 to 135 in 1e8ed70

	#' Validate bandwidth parameters
	#'
	#' This function validates function arguments related to bandwidth.
	#'
	#' @param bandwidth \code{numeric} value specifying the bandwidth to be used in
	#' calculating kernel density estimates.

This function will therefore need the cell size as an argument, so this will need to be added to the functions that call validate_bandwidth().

[mpjashby]

This is harder than it initially seems, because users can supply arbitrary polygons for the grid for which KDE values are calculated. This means that the 'cell' size has to be extracted from the grid. If this is done by finding the typical nearest-neighbour distance, it is quite slow and therefore the check should probably be made optional.