MultiModel.load_models fails on Windows

[eefm1]

When trying to load a MultiModel on Windows, a FileNotFoundError is raised:

Traceback (most recent call last):
File "C:\xxx\evaluate_model_performance_and_DoV.py", line 23, in
mm = MultiModel.load_models("biobased_test")
File "C:\xxx\polyid\polyid.py", line 541, in load_models
SingleModel.load_model(
File "C:\xxx\polyid\polyid.py", line 192, in load_model
with open(data_fname, "rb") as f:
FileNotFoundError: [Errno 2] No such file or directory: 'xx_test\model_0\xx_test\model_0_data.pk'

This issue is resolved by PR #8

[jlaw9]

Thanks for the PR @eefm1 . Looks good. I'll merge it soon.