Return element for atom key using DL_F notation.

def dlf_notation(atom_key):
    """Return element for atom key using DL_F notation."""
    split = list(atom_key)
    element = ''
    number = False
    count = 0
    while number is False:
        element = "".join((element, split[count]))
        count += 1
        if is_number(split[count]) is True:
            number = True
    # In case of for example Material Studio output, integers can also be
    # in the beginning of the string. As the dlf_notation decipher function
    # is very general in use, we have to make sure these integers are deleted.
    # In standard DL_F notation the string will never start with integer so it
    # will not affect the functionality towards it.
    # EDIT2: also the '?' atoms, you can delete them manually or somewhere else
    element = "".join(i for i in element if not is_number(i))
    element = "".join(i for i in element if i != '?')
    return element