Add tests for aggregate functions

[MaxGhenis]

This works currently, but would be good to have a test to ensure it does:

s = mdf.MicroSeries([1, 2, 3], weights=[4, 5, 6])
s.aggregate(["sum", "mean", "top_10_pct_share", "gini"])

returns:

sum                 32.000000
mean                 2.133333
top_10_pct_share     0.562500
gini                 0.204167
dtype: float64