[tei-council] revised TD for procmod

I've now made a new branch in the repo for my revised version of TD containing the processing model documentation. Some way to go yet, notably in the element specs which still seem occasionally too vague to me, and test files are still missing. But I'd appreciate some reassurance that I haven't totally misunderstood something crucial. The branch is called lb42-procmod and is based on dev as of last night