CVaRExpectation variance calculation (Qiskit/qiskit-aqua#1373)
CVaRExpectation variance calculation (Qiskit/qiskit-aqua#1373)

* partitioned some of the eval logic in cvar_measurement so that it could be re-used to compute the variance. Implemented compute_variance in cvar_expectation

* tutorial notebook runs now, need to comment, rename, and triple check the math.

* missed a term in the variance return

* small comments added

* fixed pylint and added comments everywhere.

* Update cvar_measurement.py

fixed a typehint

* typing fix

* spelling fixes

* whitespace fixes...

* added some test cases for the compute_variance method

* fixed whitespace

* docstring and edge case handling

* removed some whitespace

* formatting

* spelling
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