Derive `VectorArray`'s python bindings from `nias::VectorArray`

[tobiasleibner]

As discussed in the last meeting, we would like to have Python bindings for our VectorArray interface which directly derive from NiAS's VectorArray interface class. In order to do that, we have to

• implement all methods and a behavior similar to NiAS' VectorArray class. For this purpose, we probably need
  • an Indices class which can handle at least single integers, lists of integers, pybind11::slice types and then use this class in all methods which accept indices
  • a VectorArrayView class

[tobiasleibner]

Actually deriving our python bindings from nias::VectorArray is not possible without hacks (see pybind/pybind11#1193), so we should at least try to duplicate the functionality and keep the python wrapper as small as possible.