Various improvements to the code including but not limited to:
- Improved generation of slabs: one function does it all - slabs up to maximum Miller index, one or more specified hkl; it now also supports the frozen-core slab model and saves the created structures to a json file with relevant metadata; has better support for custom VASP input sets
- Additional convergence tools: Fiorentini-Methessel and Boettger surface energies calculated automatically with new electrostatic potential tools added; added parsing of structures and bond lengths for convergence
- Clearer and more intuitive plotting for surface energies
- Expanded tutorials