metadata validation

[c-bata]

For example

class SampleDataFrameValidator(pv.DataFrameValidator):
    row_num = 5
    column_num = 2
    label1 = pv.IntegerColumnValidator('label1', min_value=0, max_value=10)
    label2 = pv.FloatColumnValidator('label2', min_value=0, max_value=10)
    metadata = {
        'metadata1': str
        'metadata2': int
    }