Make installable

[jthorton]

As the first step to getting the package on conda-forge (#13) I have made it installable which involved some reorganisation of the files into folders which made sense to me but very happy to change them if needed. I have also exposed a helper function to make it easy to use the models from python once the package is installed, see the README.md updates on how this works.

Questions

• Do we need to expose the ASE and pysisyphus optimisation scripts as CLI entry points?
• Should we have a simple CI test which ensures the models can be loaded and that the calculators work as expected?
• Should we expose the ability to load the individual models not just the ensemble models?

[isayev]

@jthorton I just created a development branch let's move over there