Improved sparse getindex

This PR improves the sparse getindex performance.  Execution time is shorter by a factor of 1 to 500 on the (somewhat primitive) examples I ran, memory usage is down by a factor of 1 to 5000.  The performance
tests and results are here: https://gist.github.com/mauro3/7f01931b5f25d7b397af


Updates:
- row indexing with ranges has now a dedicated method (biggest
  speedup & memory gain).
- row indexing with sorted vectors is now using a method similar to
  the original getindex_general (some large speed-ups/memory gains as well)
- row indexing with unsorted vectors uses original getindex_general
  (therefore no gains)
- I had to add some dis-ambiguity methods
- more tests added.  --code-coverage shows full coverage of all
  getindex & related functions