Add XTB optimizer structure editor

It could be very useful, for example when generating molecules from smiles, to add the posisbility of optimizing a structure with xtb this can be done as follows:

(see https://github.com/pgasparo/smiles2ase also for one example)

suppose mol is an ase structure:
```python
from xtb.ase.calculator import XTB
mol.pbc=False #needed
mol.calc=XTB(method="GFNFF")
## example on how to fux atoms below:
from ase.constraints import FixAtoms

fix_z=max(mol[np.where(np.asarray(mol.get_chemical_symbols())=="Au")[0]].positions[:,2])
idatoms_tobefixed = np.where(mol.positions[:,2]