Fix filepath to allow running from any directory #50 [skip modfiles]

Before fix, when running from 421xxx/tmp_421999 (which is where the CI job would run from):

IOError: [Errno 2] No such file or directory: '421xxx/421999/mc.Sh_228_Wmunu_EnhLogPtV.py'

After fix:

OK: file does not contain includes pointing to MC15JobOptions

NB before merging @pengc should check (or provide instructions for checking) that this fix is ok to be transferred to athena

Closes #50 (closed)