Bind HLL get_composite_estimate

[jaideeppyne]

Fixes #68.

get_estimate() is the HIP estimator. It is the more accurate of the two, and it can change with insertion order. The composite estimator is order-independent, and it is what a union uses once HIP can no longer be kept. C++ and Java already expose it. This binds the same method on both hll_sketch and hll_union.

The docstring matches the C++ note that this is the estimator for when order-independence matters, not a replacement for get_estimate().

The test builds the same items in opposite orders and checks that get_composite_estimate() agrees, and that hll_union.get_composite_estimate() matches get_result().get_composite_estimate().

Tests: python -m unittest tests.hll_test.