#124 Operator cleanups

[Ulf Lorenz]

Some things that could/should be done:

• Would it be possible to get rid of the SchroedingerOperator? Ideally, you would never wrap a primitive explicitly in a SchroedingerOperator, instead there is some implicit type conversion. Then, the complex operators could be named Liouvillian, and there is less confusion on the user's side.
• Propagators could get a single constructor that takes an OperatorMatrix. If the OperatorMatrix allows implicit conversion from a std::shared_ptr<linearoperator>, one could get rid of the explicit constructor for that type.</linearoperator>

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No simple way to get rid of the SchroedingerOperator, and there are sometimes differences between using a LinearOperator and an OperatorMatrix.
Instead, I went for some renamings to make the use of the individual classes clearer. In particular:

• OperatorPrimitive => Operator
• LinearOperator => Expression
• OperatorMatrix => EquationSystem
• SchroedingerOperator => SchroedingerEquation