Add aggregate support

[kdorr]

mpl-altair currently doesn't support conversion for Altair charts that use aggregations.

Code for reproduction

data:

df = pd.DataFrame({
    'a': [1, 1, 2, 3],
    'b': [1.1, 2.2, 3.3, 4.4]
})

Example 1:

chart = alt.Chart(df).mark_point().encode(
    alt.X('a'), alt.Y('average(b)')
)
mplaltair.convert(chart)

Example 2:

chart = alt.Chart(df).mark_point().encode(
    alt.X('a'), alt.Y('count()')
)
mplaltair.convert(chart)

Actual outcome

A NotImplementedError is raised for anything that has aggregated data.

Expected outcome

Any plot with aggregation should work.

The first chart should look something like this:

The second chart should look something like this:

Possible solution

During the initial altair chart parsing, manipulate the data to include the aggregations. See parse_chart.ChannelMetadata() (specifically _aggregate_channel()).