This method mimicks the behavior of normal vector access with square brackets like, e.g., v[5] = 1.
The problem is that the return type is different for each value of the argument in the brackets.
Therefore we implement a trick using std::integral_constant.  To access the first entry of
a MultiTypeBlockVector named v write

  MultiTypeBlockVector<A,B,C,D> v;
  std::integral_constant<int,0> _0;
  v[_0] = ...

The name '_0' used here as a static replacement of the integer number zero is arbitrary.
Any other variable name can be used.  If you don't like the separate variable, you can writee

  MultiTypeBlockVector<A,B,C,D> v;
  v[std::integral_constant<int,0>()] = ...