#154 EquationSystem has a constructor that fills the diagonal with an Expression

[Ulf Lorenz]

What and Why

Currently, propagating a set of wave functions is somewhat cumbersome, because you have to propagate each state manually. There is functionality for doing everything in one go in the form of EquationSystem, but it is even more annoying to set up than propagating the states manually.

A solution would be to add a constructor that gets an expression and a size, and sets up a diagonal equation system with only this expression. Then you could conveniently propagate all wave functions in one go.