Pyxtal

This is a major release

- Now the structure generation will be called from PyXtal
- enable two styles for cif output
- polish the structure perturbation function
- enable the save/load dictionary function for pyxtal, pyxtal_molecule, atom_site, mol_site
- simplify the Group class
- add list_wyckoff_combinations

- reorganized the symmetry code
- fixed a bug in lattice mutation
- reorganized the t-subgroup function
- fixed a bug when the site is given
- added the seed function to load ase/pymatgen structure

- fixed some bugs in subgroup function
- updated the tutorial for atomic crystals

Added an initial support on the subgroup function.

- Switch the travis support to GitHub
- constrain the molecular center to the unit cell when exporting the structure to ASE/Pymatgen
- add the reading force function to the gulp calculator.

- Improved the visualization of low-dimensional systems
- Added working examples