SMTG-UCL/surfaxe: v2.0

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