#160 Make expectation value observers more usable

[Ulf Lorenz]

What and Why

In the demo MolVibration/OH/2, you can see some deficit of the expectation value observer. Each takes only a single operator and accumulates the data, without writing it to a file. This leads to some awkward coding where you store all the observers and retrieve the data in the end.

This calls for a better solution that should be devised here. There are principally two solutions:

• The observers directly write out the data to a file
• The observers can calculate multiple expectation values.

The first option is less attractive, because it couples different types of code (propagation observer and file access) for a special use case. So a better suggestion seems to calculate all the expectation values with a single observer.

Acceptance criteria

• An ExpectationValueObserver takes a list of operators and calculates the expectation values for each.
• The demo MolVibration/OH/2 uses this scheme to simplify the output