Made static const in MSLayersAtAngle

The file local static doPrint never changes and is not meant to be
seen outside of the file. Therefore declared the variable static const.
Even if there was logic to change value of doPrint at run time, it
wouldn't work well with the threaded framework.
Merge pull request #3828 from Dr15Jones/makeStaticConstMSLayersAtAngle

Multithreading -- Made static const in MSLayersAtAngle