xyticks: Creates the ticks for the maps.
In fernandomayer/FishMaps: Plots Fishery Data into Maps

Creates the ticks and labels to print in the maps. This is an internal function.

1	xyticks(xlim, ylim, square)

`xlim`	The x limits for the map. Usually this will be the longitude limits.
`ylim`	The y limits for the map. Usually this will be the latitude limits.
`square`	Size of the square. It must be a length one numeric vector (e.g. for squares of 1x1 degrees, this should be 1). Roughly speaking this can be viewed as the resolution of the map.