requirements: Clean up the API of TmfAnalysisRequirement

requirements: Clean up the API of TmfAnalysisRequirement

* Remove the 'fType' field in favor of sub-classing
* Remove the possibility to merge requirements, a composite requirement will
be added in a later patch
* Have the priority level be for the whole requirement instead of per-values
* Values cannot be added except at the constructor
* Remove requirement helper class and tests: the methods there referred to the
type which does not exist anymore.

Change-Id: I01897cd705452833cbdbe6c57dd819a5a1ecfd22
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>