Address ModelToIdf output files erroneously deleted during job cleanup

@elainethale please review this patch with respect to #424 

I believe your environment in one instance was causing the drive letter to be lower case, but once you entered the debugger the environment was changed enough to make the drive letter upper case again. 

I am unable to reproduce this on Linux or Windows.

I've made the job cleanup process case-insensitive on all platforms, erring on the side of caution.

Also, please verify the instance on which you have seen this problem on Linux (preferably with logging output as per the Windows instance of the failure).

-Jason