Integrate further into ASE

[chpxu]

CONQUEST has an ASE interface. To allow interoperability, make the Atom class inside conquest2a.conquest have a method to return an ASE Atoms object. This requires the following adjustments:

• Allow conquest2a's Atom to be converted to and from ASE Atoms
  • Make the lattice parameters part of the Atom class, like in ASE's Atoms.cell
  • Convert Atom to ASE's Atom, then construct ASE's Atoms from this list
  • Modify code to work with new conquest2a's atoms