Improve dealii-9999.ebuild to be more consistent with Gentoo's best practices

Specifically:

  - Rename use flags thread -> tbb, umfpack -> sparse as match common practice
  - Add virtual/pkgconfig to DEPEND
  - Remove double occurence of sys-libs/zlib
  - Provide a static-libs use flag
Clean up the documentation installation: Only install html files if +doc is set