- Doxygen was creating one svg for every class, showing in which library it resides. This created
  6000 svgs, which were mostly identical. Now, only ~100 are created for every library in ROOTSYS/lib/,
  and the relevant webpages link to those.
  This speeds up the "listlibs" step of the doxygen process signficantly.
- Add parallel processing for collaboration diagrams of class webpages.
- Remove scanning of rootmap files. It doesn't work, any more. TCling needs
  a fix for ROOT-10900, so gInterpreter->GetClassSharedLib() will always
  return the right thing.
- Further, parallelise the modification of class webpages.