- Redo the object model and inheritance
- Make goot PHPUnit tests

* toCML methods DONE
* residue class DONE
* macromolecule class DONE
* protein class ? DONE
- nucleic acid class ?
* PDB atom class DONE
* PDB molecule class DONE
- CML file class
* XYZ file class DONE
* PDB file class DONE
- reading from compressed files