proposed enhancement: interface and docs for a programmatic use (without cli)

[tdaff]

Since the backends are standard, this is probably already possible. However, some documentation and examples would be good to show that abcd can be used within scripts to interact with ase objects. It could be something like:

import abcd

my_db = abcd.Database('something.db')

for atoms_object in my_db.query('energy<-3460.93272745'):
    new_atoms = do_something(atoms_object)
    my_db.store(new_atoms)