Move some template code from diagnosticsDockWidget.py to uvcdat.py.
(originally done August 5, 2014)
Change sorting function for plot sets to a natural sort defined in the diagnostics package.
This will put plot set 10 after plot set 9, and we can tinker with it as necessary.
(originally done August 6, 2014)
    Fix a serious bug in the diagnostics GUI where it mattered whether you clicked on Obs1 or Data1 first.
    What I did was to separate the filter for obs from the filter for "data" (i.e. model output).
    A consequence is that the Options object is not really needed for the call of dirtree_datafiles,
    but I left it in for the moment.
    Note that this is still buggy.  It will be buggy until the filter, if any, and the path are
    inextricably bound together.  Thus, for example, the obs2 filter and obs1 filter would be
    different and bound together with the obs2 path and obs1 path, respectively.
    Moreover, this code will be unnecessarily complicated as long as there is such a strong
    internal distinction made between obs and model output.  They should and can be all
    handled with a single method.
(originally done August 6, 2014)
    Fix bug in check for whether obs exist.
    In displayCell, check whether a plot result object (res30) has presentation=="text".
    That means it's a text table, so we can't plot it.
(originally done September 8, 2014)