PLRsearch: Initial implementation and suites

Missing bits:
- Add up-to-date .rst document (in child Change).
- Prepare for releasing to PyPI.org (in child Change):
-- Either copy dependencies from MLRsearch, or list in requirements.
-- Perhaps move common dependencies to separate package
   for both search to depend on.
-- All the other metadata stuff.

Jira: CSIT-1276

Change-Id: I277efdb63dbb90b30e11f4e30a82e2130ac8efc3
Signed-off-by: Vratko Polak <vrpolak@cisco.com>