Backend option in Compound.save()

[rsdefever]

Describe the behavior you would like added to mBuild
All the user the option to specify the backend for saving files.

Describe the solution you'd like
Add a backend option to Compound.save in analogy to the mbuild.load function.

[umesh-timalsina]

I think the save function tries to use various backends based on file extension right?

[daico007]

@umesh-timalsina That should be the case now. I think the backend keyword may be helpful for selecting between parmed writer vs mdtraj writer?

[CalCraven]

Do we have a from_traj conversion? If so, you could just directly load into mdtraj and then convert to mBuild. But this is an easy addition to the loader as well.