bug fix for "_bmark_cache.npz file stores at program path"

[diphosphane]

_bmark_cache.npz file stores at program path will result several problem:

1. if user install sgdml with system python (/usr/bin/python3), then the path /usr/lib/python3.*/site-packages/sgdml is not writable, which will results problem.
2. if users install sgdml in a cluster/workstation, and execute multiple tasks simultaneously, then _bmark_cache.npz will be overwrite multiple times. It will result verify fail or output error when one task read the wrong _bmark_cache.npz which is written by other task. This problem will also happened in cluster, when sgdml is installed in cluster software path, and calculation node uses this software path.

In my fix, I modify the _bmark_cache.npz path into current task execution path, it would solve this bug.

I only test it in my task, I'm not sure whether this modification will introduce other bug or not.

Because this modification related with file path, I also checked the python code os.chdir (which may conflict with my modification) , and not found the usage, so I think it may not introduce bug.