Feature: Improve usability of aggregate by allowing more then one aggregation function

[timo-at-lanalabs]

This is the way it currently is:

df1 = api.aggregate(log_id, metric="duration", aggregation_function="min")
df2 = api.aggregate(log_id, metric="duration", aggregation_function="max")
df3 = api.aggregate(log_id, metric="duration", aggregation_function="mean")

Could we allow for more than one metric?
df1 = api.aggregate(log_id, metric="duration", aggregation_function=["mean", "min", "max"])