helper function to check the input of binning

Usage

.check_binning(bin_param, bin_type, w_x, w_y)

bin_param: A numeric vector indicating the size of the bin. If the bin_type is "square" or "rectangle", this will be a vector of length two giving the numbers of rectangular quadrats in the x and y directions. If the bin_type is "hexagonal", this will be a number giving the side length of hexagons. Positive numbers only.
bin_type: A string indicating which bin shape is to be used for vectorization. One of "square" (default), "rectangle", or "hexagon".
w_x: A numeric vector of length two specifying the x coordinate limits of enclosing box.
w_y: A numeric vector of length two specifying the y coordinate limits of enclosing box.

the length of total bins