Selbox londim and latdim work with flexible dimension names

Hi @aho,

In function Selbox, parameters 'londim' and 'latdim' from here are described as integers for the lon and lat position. However, if they are set to NULL, they are set as fixed names: 'lon' and 'lat' as in line 50 and 62.

I think it would be better if they are londim = 'lon' and latdim = 'lat' by default and they can accept different spatial coordinates names. What do you think? I can develop it if you want.

Eva