Add option to traverse selected translation units only once for multiple diagrams

[bkryza]

Currently, translation units specified by glob patterns are unique to each diagram, i.e. for each diagram specified translation units are traversed separately.

This can be very inefficient for cases when we want to generate a large number of diagrams from a large number of translation units (or in particular from all translation units in a project) and then generate diagrams from a single large intermediate model.

The option could be for instance:

share_translation_units: true

in which case the glob pattern can be specified at the top level of the configuration file and each diagram will use the entire intermediate model from these translation units and the include/exclude filters will be applied only during diagram generation phase (e.g. filters will be applied only to diagrams with (is_completed() == true).