Fixed statics in SiStripDetInfoFileReader

The static analyzer was complaining about two file local statics in SiStripDetInfoFileReader. One I made into a stack variable which probably makes the routine faster and the other I made const.
Merge pull request #1882 from Dr15Jones/fixStaticsInSiStripDetInfoFileReader

Multithreading fixes -- Fixed statics in SiStripDetInfoFileReader