calc_expectation but for labelled distributions

[sbenthall]

calc_expectation returns the expected value of a function over a distribution.

https://github.com/econ-ark/HARK/blob/master/HARK/distribution.py#L2298

It isn't yet implemented to be compatible with Labelled distributions: https://github.com/econ-ark/HARK/blob/master/HARK/distribution.py#L1450

It probably can be written to wrap expected: https://github.com/econ-ark/HARK/blob/master/HARK/distribution.py#L2422

See also #1483