added a class to get physical and planetary constants. bits are from R. Pierrehumbert climate book utilities
separated dimensions + index retrievals. this allows e.g. to get pressure coordinates at the point xy considered for profiling
added the possibility to changetime with correctls_noadd to NOT shift to a monotonic axis and simply get Ls as it is in the file
added () to a formula in planets
added functions in ppcompute to easily calculate derivatives
made ccol an attribute in ppplot
made negative and xkcd methods rather than global functions (now no need to edit sources to make it active)