Calculate aspect ratio for plotting geographic maps; create nice degree axis labels

mapasp(data, xlim, ylim)
degreeLabelsEW(x)
degreeLabelsNS(x)

Arguments

data	object of class or extending `Spatial`
xlim	the xlim argument passed (or derived from bounding box)
ylim	the ylim argument passed (or derived from bounding box)
x	numeric; values at which tics and marks will be generated

mapasp is used for the aspect argument in lattice plots and spplot;

let x = dy/dx, with dy and dx the y- and x-size of the map.

let s = 1/cos((My * pi)/180) with My the y coordinate of the middle of the map (the mean of ylim)

for latlong (longlat) data, mapasp returns s * x. for other data, mapasp returns "iso".

the values for x are typically obtained from axTicks