v0.0.3-beta

This PR adds some updates and improvements into the code.

List of changes:

• Now the building block file structures are saved in the save_dir folder;
• The Tools.elements_dict module has new features:
  • full_name
  • atomic_number
  • atomic_mass
  • polarizability
  • pauling_electronegativity
  • thermo_electronegativity
  • mulliken_electronegativity
  • sanderson_electronegativity
  • allen_electronegativity
  • ghosh_electronegativity
  • martynov_batsanov_electronegativity
  • atomic_radius
  • covalent_radius
  • vdw_radius
• The Tools now has the capability to save in the qe format
• Tools.get_kgrid now returns integers
• The gjf now are json files, as a result of #4, #33, #28
• print and repr methods are implemented
• SMILES codes are available on BuildingBlock.smiles
• Save functios were remodeled. Now it is possible to use BuildingBlock.save() and Reticulum.save() to save the structures;
• New linear building blocks were added as a result of #29
• Add new building blocks with carbenes as result of #34
• Change the Building Blocks symbols as result of #36