add gridfile to commandline options #54

Summary

It should be possible to select a grid file when opening a file with psy-view

Reason

Often coordinates are saved in a separate file. psy-view should be able to open it without the need to merge two netCDF files. Ideally we have this available in the command line, but also the possibility to choose the gridfile from inside the GUI.

Examples

• psyplot.data.open_dataset already has a gridfile option.