major update for CombineTools code

New features:
 * Parsing and writing datacards that use pre-built RooWorkspaces, e.g.
 hgg. "param" datacard lines also supported and deduced automatically
 when writing. Observations may now link to a RooAbsData object, and
 processes to a RooAbsPdf object.
 * More robust datacard writing in general, new function to write
 histograms into an already-open root file
 * Started work on datacard creation tools: Observation and Process
   creation functions done & tested