Bandstructure plots for large systems

[janssenhenning]

At the moment there are two problems with bandstructure plots with a large number of bands especially:

• The points outside of the plotting area still contribute to the overall size of the saved pdf/png (matplotlib) especially for weighted bandstructures. This can be solved by excluding the points outside the plot area at the point where the limits are known
• The reading in of all bands wastes a lot of memory/time if the user is only interested in a very small region around the fermi energy. This should be solved by introducing a way to exclude values immediately after (or even while, maybe h5py has some way of doing that) reading the datasets in the HDF5Reader