Introduce superstrate and substrate

[dschick]

The toolbox already provides a quasi-static substrate of the Structure objects, which is essential e.g. for hard x-rays scattering (XrayDyn, XrayKin) but is neglected in other simulations.
For other light scattering simulations, it is important to have a semiinfinite medium above (superstrate) and below (substrate) of the actual sample to give the correct results, see. e.g #96

In that sense, it will be beneficial to introduce a Structure.superstrate as a top layer for each structure.
Both the superstrate and the substrate should be vacuum on initialization.
Light scattering simulations such as heat.get_multilayer_absorption and XrayDynMag as well as GTM should work with that.

Also when reversing the sample structure by Structure.reverse() the superstrate and substrate must be taken into account.