Non-standard but schema-valid rId values raise exception on Presentation.open()

[scanny]

Jasper Reports, and perhaps other PresentationML generators, produce relationship id values, some of which do not follow the PowerPoint® client convention of 'rId%d'. In this particular case, the relationship ids included 'rIdSm' for the slide master. A ValueError was thrown in _Relationship._num when this value was encountered.

[scanny]

Close inspection of the relevant schema reveals that rId values may be arbitrary xsd:ID strings, starting with a letter or underscore and containing only letters, digits, underscores, periods, and dashes. Renumbering of relationship ids was commented out for now and non-standard rIds are preserved intact through the open() to save() cycle.