ENH: Added util functions

[deshmukg]

• Closes Create util function to get selected bulk, surface, and adsorbate atoms from a slab #3. Closes Create util function to featurize bulk, surface, and adsorbate graphs #8.
• Added utility functions 'partition_structure' and 'featurize_atoms' to partition a catalyst structure into bulk, surface, and adsorbate atoms and featurize those atoms respectively.
• Added a 'ReactivityFeaturizer' that featurizes atoms with atomic number < 20 with valence electrons and others with surface d-band center.