Update main_gsm.py

[jandestrada]

Please make this change. Using pip install git+https://github.com/ZimmermanGroup/pyGSM.git' or poetry add git+https://github.com/ZimmermanGroup/pyGSM.git` leads to the following import error:

  from pyGSM.growing_string_methods import DE_GSM
  File "[path-to-pygsminstallation]/pyGSM/growing_string_methods/__init__.py", line 2, in <module>
    from .main_gsm import MainGSM
  File "[path-to-pygsminstallation]/pyGSM/growing_string_methods/main_gsm.py", line 15, in <module>
    from molecule.molecule import Molecule```

## Description
Will allow for bugless installation when using pip or poetry to install this package instead of having to git clone repo.