#199 Some small usability improvements

[Ulf Lorenz]

Minor issues that should be relatively easy to fix, but avoid confusion or extra work on the side of the user:

• make sure that Eigenstates returned by EigenstatePrimitives are always real; diagonalization can return complex states
• TensorState could get a constructor that gets a (1D) wavepacket::State as input; this simplifies usages such as in the Interferometry demo.
• project() could get a variant that does not take a projection operator, but an index and projects onto the corresponding channel