Protein mutation -- Allow optional small molecules in system generation Modeller

[ijpulidos]

Currently, the system_generation.get_omm_modeller requires a dictionary with the small molecule components. In order for this same function to be used for the protein mutation protocols (and other protocols), we probably want to make the small_mols argument an optional one.

Now, that would make all the component arguments to be Optional, which I think it's fine. That is, this function would allow users to generate an empty OpenMM modeller object if they don't pass any component as an argument.

A workaround this is using an empty dictionary for the small_mols argument.