gFEX MET fix
With this MR, a few bugs or inconsistencies in the gFEX MET implementation are fixed as follows:
- The mask applied for extracting MET quantities in the decoder was updated with the correct value 0x00FFFF
- A different resolution (800 MeV rather than 200 MeV) used in MET algo when applying thresholds from the trigger menu is used
- Updating the overflow checks for MET quantities both in simulation and in the decoder