Also use "lon" and "lat" for PWS and add optional "x" and "y" for projected coordinates

[cchwala]

Two things could be improved regarding coordinates:

1. We use site_0_lat etc for CML and SML. It would be good to also use the short form "lon" and "lat" for PWS, since the full name can be given in the attributes. Since the convenctions for PWS should also be used for any other gauge data in our work, it will save some typing and and results in shorter code when using gauge data.

2. Since we need projected coordinates to easily calculate distances, needed to find neighboring sensors and for spatial interpolation, I suggest to add x and y ass optional coordinate variables to have a definition how to store the projected coordinates. For SML and CML it would be site_0_x etc.