TMF: Add default constructor to experiment and method to initialize it

TMF: Add default constructor to experiment and method to initialize it

The experiment behavior is thus equivalent to that of the trace and we can
eventually instantiate an experiment in the UI, just as we can a trace, using
an experiment constructor from a configuration element.

Change-Id: Ib7957a3b6458e424bd34d82d4757174233edec04
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/20666
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
IP-Clean: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>