update comments and abbreviations
added diff_id method to class plotspec, and the class name now can be abbreviated as ps.
For AMWG plot set 4, use new dict_id method of plotspec class.  I haven't tested this yet.
When it works I'll do the same for plot sets 5&6.
Implement full-featured (i.e. tuple) ids for the plotspec class, in plot sets 4 and 5&6.
This gets ids from the variable the plot plots, using the new method dict_idid().
Allow a plotspec to be initialized with a tuple id, not only string.
Implement a new method plotspec.dict_idid() which converts the (tuple) id of a variable
to a proposed id for a plotspec.
back to testing on plot set 4