Add a column to a FITS file.

    ADW: Could this be replaced by a ftool?

def add_column(filename,column,formula,force=False):
    """ Add a column to a FITS file.

    ADW: Could this be replaced by a ftool?
    """
    columns = parse_formula(formula)
    logger.info("Running file: %s"%filename)
    logger.debug("  Reading columns: %s"%columns)
    data = fitsio.read(filename,columns=columns)

    logger.debug('  Evaluating formula: %s'%formula)
    col = eval(formula)

    col = np.asarray(col,dtype=[(column,col.dtype)])
    insert_columns(filename,col,force=force)
    return True