check whether syst are there; report missing ones
DiLeptonMatrixMethod::configure : cleanup and simplify

Details:
define a RetrieveOrNull struct to simplify the histogram retrieval and
bookeeping
DiLeptonMatrixMethod::getStatError: clarify
FAKE_REGION, FRnames: add new control regions

Details:
- add the new control regions
- within the string array, put comma on newline (easier to spot missing one)