VTL - Aggregate operators

Name	Symbol	InMemory	Spark
Aggregate invocation		✔️	✔️
Counting the number of data points	count	✔️	✔️
Minimum value	min	✔️	✔️
Maximum value	max	✔️	✔️
Median value	median	✔️	✔️
Sum	sum	✔️	✔️
Average value	avg	✔️	✔️
Population standard deviation	stddev_pop	✔️	✔️
Sample standard deviation	stddev_samp	✔️	✔️
Population variance	var_pop	✔️	✔️
Sample variance	var_samp	✔️	✔️