ENH: custom atoms-arrays in database

Many calculators and parsers introduce custom arrays in ase.Atoms. These are not yet supported in ase.db.

This merge request introduces a small way of achieving the same.

Changes:

• The row object writes the custom arrays into its _data
• When row.toatoms method is called, it checks its _data for custom arrays and adds them to atoms object
• When writing new db entries, the data is updated with row._data, to ensure custom arrays are passed
• When updating db with an atoms object the custom arrays in oldrow are popped.

This merge request also adds unit test for the same.