refactoring scan.py (still in progress)
removed old, unused corr.py for calculating correlators
removed old method for calculating dmetal energy
old_measurements is really composite_measurements ... we might bring these back.
removed ElectronRingExchangeMeasurementPlan, which has been superceded by SingletRingExchangeOperator
moved measurements from pyvmc.core.measurement to pyvmc.measurements
move operators to pyvmc.operators
removed all old workflow stuff, including twisted dependency.  disabled 1d renyi test in the process