Allow optional arguments via VarParsing

[IzaakWN]

By allowing optional arguments via VarParsing, user can run ispy_10_X_X_cfg.py without needing to edit the file, e.g.

cmsRun python/ispy_10_X_X_cfg.py inFile=root://cmsxrootd.fnal.gov//store/data/Run2018B/Charmonium/AOD/17Sep2018-v1/120000/3ADC2E5A-4F1B-4546-8CB6-58DDBBC921D3.root maxEvts=10
cmsRun python/ispy_10_X_X_cfg.py inFile=file:AOD.root maxEvts=50

The defaults are the same as before.

Also, I made the file executable with chmod u+x, and added the shebang

#! /usr/bin/env cmsRun

so one can run the script as

python/ispy_10_X_X_cfg.py