first vector arrays in C++

[ftschindler]

We need at least:

• indices on the C++ side
• a vector array view on the C++ side
• the vector array interface on the C++ side
• one implementation (say list-based) on the C++ side

Possible follow-ups are (separate issues once we pick them up):

• Given a class MyVectorArray : public VectorArrayInterface on the C++ side, how do we ensure the corresponding Python object is derived from nias.interfaces.VectorArray?

  Ideally, the bindings would directly inherit on the Python side, but there might be issues in pybind11 (see Subclass python class in C++ pybind/pybind11#1193).

• Given any Python vector array derived from nias.interfaces.VectorArray, how can we use that in bindings of C++ functions?

  Say, we have an algorithm implementation on the C++ side like

  double my_algorithm(const VectorArrayInterface<double>& vector_array);

  we want to bind that so that we roughly have

  def my_algorithm(vector_array: nias.interfaces.VectorArray) -> float:

  on the Python side.

  This likely requires a wrapper on the C++ side that holds the Python object and implements the VectorArrayInterface.

[tobiasleibner]

See nias-project/nias-cpp#10 for the initial implementation on the C++ side.