Code does not compile on OS X

[mphoward]

There are some issues (likely with the version / clang-iness of the compiler). I noted the following so far and will look into further:

• Mesh needs a virtual destructor
• The interpolate method will not compile because it is believed to lead to infinite recursion. I was confused about how this compiled on GCC. We need to explicitly cast the view to a const view so that we can chain these methods. This means we need a copy constructor that can handle const and non-const types. That will also mean adding a default copy constructor and maybe move constructors.