Python 3 error loading pickle

[cescgina]

When using pmx from the develop branch with python3 I get the following error:

TypeError: a bytes-like object is required, not 'str'
The error occurs in the following line:

pmx/src/pmx/library.py

Line 65 in 1063622

return pickle.load(open(data_file))

The fix seems quite easy, just open the file with mode "rb" and pass bytes to pickle.load, and that seems to work both in python2 and 3. I can open a PR for you to review if you want.