[FEA] aggregation should be more opaque

[jrhemstad]

Is your feature request related to a problem? Please describe.

The aggregation class is used for specifying details of what kind of aggregation a user wants from APIs like groupby. User's should not need to interact with this class other than using the make_*_aggregation APIs to get an aggregation object.

Originally this type was opaque, but if I remember correctly, that broke Cython. So we had to make it non-opaque. However, this has lead to issues of users using methods of aggregation that they should not.

Describe the solution you'd like
aggregation should be made more opaque. The PIMPL pattern is probably the right call here such that the type's definition is still visible to appease Cython, but it's members aren't available.

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