Warn if nans exist in a numpy array.

def warn_if_nans_exist(X):
    """Warn if nans exist in a numpy array."""
    null_count = count_rows_with_nans(X)
    total = len(X)
    percent = 100 * null_count / total

    if null_count > 0:
        warning_message = \
            'Warning! Found {} rows of {} ({:0.2f}%) with nan values. Only ' \
            'complete rows will be plotted.'.format(null_count, total, percent)
        warnings.warn(warning_message, DataWarning)