Flexible coordinates

@shresthp @thober

now the dimensions are named in dependence of the input:

case 1: for latlon 1d, dimensions are equally named lat/lon

case 2 for latlon 2d, easting/northing are additionally read in and dimensions are named easting/northing

case 3: if neither latlon 1d or 2d coordinates are provided, the output dimensions are x/y

I switched back to x/y from ncols/nrows. There was a mix up with ncols/nrows referring to x/y coordinates, which complicated the understanding of the code.

Please test the code and have a careful look at the output. I couldn't run the check cases, because of a missing makefile..