Remove (empty) default constructor for ArrayView

[masterleinad]

All the member variables for ArrayView are currently declared const. Hence, an empty constructed object can never point to anything useful.

[bangerth]

I suspect that we don't want std::vector<ArrayView> to work, or similar. Or do we?

[davydden]

./run-tests

[drwells]

I think that we should keep this so that ArrayViews are default constructible. It might be useful to put them in containers later on.

[masterleinad]

It might be useful to put them in containers later on.

That does not require a default constructor necessarily (only if you want the objects to be default initialized).

I am not opposed to allow to modify the member variables if we decide to do so. I am just not happy that we are somewhere in between by having a constructor that always creates an object that can never be used.

[drwells]

Its been merged, but I checked and this is okay. We can do things like

  double a = 0.0;
  std::array<ArrayView<double>, 2> views2 {{{&a, 1}, {&a, 1}}};

which is, I think, the extent of what makes sense to do with this class.