Restore compatibility with original openHLT codes

- openHLT.py now should work as expected as the original openHLT.py.
- isData boolean can be turned off via command line.
- ifiles can be fed via command line.
- hlt_MET_dev_skimHLTPFMET150.py contains paths that require HLTrigger-JetMET package, while hlt_MET_skimHLTPFMET150.py does not.
- hlt_MET_skimHLTL1ETM40.py is added.
- README.md is updated.

Note that the codes are not yet validated.