minor, provide some freq counting routines in corpus lib

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, provide binary idf(1) to count term frequencies over different documents

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, allow to pass term ids as arguments, where the frequency profile will be printed in return

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, allow passing in tids on stdin for the corresponding freq profile to be printed

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, open freq corpus in RDONLY mode if query mode is used

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'feat/idf'

* feat/idf:
  minor, open freq corpus in RDONLY mode if query mode is used
  minor, allow passing in tids on stdin for the corresponding freq profile to be printed
  minor, allow to pass term ids as arguments, where the frequency profile will be printed in return
  minor, provide binary idf(1) to count term frequencies over different documents
  minor, provide some freq counting routines in corpus lib