'Accesses data associated with the specified field in the box collection.
Args:
field: a string parameter used to speficy a related field to be accessed.
Returns:
a numpy 1-d array representing data of an associated field
Raises:
ValueError: if invalid field'

def get_field(self, field):

    if (not self.has_field(field)):
        raise ValueError('field    {}    does    not    exist'.format(field))
    return self.data[field]
