use of base_columns should be allowed as alias for base_column with multiple base_columns

[ronanstokes-db]

Code is present already to do this but does not work for following code snippet:

import dbldatagen as dg
from pyspark.sql.types import StructType, StructField, StringType

shuffle_partitions_requested = 8
partitions_requested = 8
data_rows = 10000000

dataspec = (dg.DataGenerator(spark, rows=10000000, partitions=8)
.withColumn("name", percent_nulls=1.0, template=r'\w \w|\w a. \w')
.withColumn("payment_instrument_type", values=['paypal', 'visa', 'mastercard', 'amex'], random=True)
.withColumn("payment_instrument", minValue=1000000, maxValue=10000000, template="dddd dddddd ddddd")
.withColumn("email", template=r'\w.\w@\w.com')
.withColumn("md5_payment_instrument",
expr="md5(concat(payment_instrument_type, ':', payment_instrument))",
base_columns=['payment_instrument_type', 'payment_instrument'])
)
df1 = dataspec.build()

df1.display()