Experiment manager bug fix

[Conradcon]

Reviewed the experiment manager issue #135 it seems that the issue was in the execute_runs() function in the experiment_manager.py file.

The function is trying to call ponyge.py as an executable string in the subprocess call, but can't find it as it's not in the current working directory.

 exec_str = executable + " ponyge.py " \
               "--random_seed " + str(seed) + " " + " ".join(sys.argv[1:])

    call(exec_str, shell=True)

I added a call in the experiment_manager.py main() function to change the directory to the parent dir chdir("..") that allowed ponyge.py to run and resolved some file path issues for grammars and datasets.

I also added the parent directory to the path with pathlib as only the current working directory scripts got added when experiment_manager.py is run.

from pathlib import Path
path.append(str(Path().resolve().parents[0]))

I also updated some module imports that didn't work with the new directory structure.

I ran the example problems as tests with experiment_manager.py and with the standard ponyge.py on a Macbook running Python 3.8.5 with no issues.