Various improvements to the cli and the backend

Add support for a number of new options:
  --inmemory to preload the data in memory (makes it possible to process
large text files)
  --floats-only to parse int-looking fields as floats too
  --trainwith (obsoleting --testset) to specify a separate training set

Change the test/train semantics: the default set is now the testing one.
Merge branch 'master' of github.com:cpitclaudel/6.830