Filename loader

For when mdtraj just doesn't cut it :)

also added the current directory to sys.path when loading the estimator from a pickle file. This lets you define a new estimator in a module in your run directory
can you do this in a context manager so that it's not a permanent change to `sys.path`? (e.g. like https://github.com/rmcgibbo/osprey/blob/master/osprey/utils.py#L28-L36)