Fix for build issue with fundamentals-differentiation tutorial code

This MR provides a fix to the build error with the fundamentals-differentiation tutorial code that was occurring when using newer compilers (e.g. on Mac OS Mojave).

The build error no viable overloaded '=' was occurring when trying to assign the result of accessing the PointsManager in the code in tutorials/fundamentals/differentiation/fundamentals-differentiation.

Working with master, building the fundamentals-differentiation tutorial code as part of a full Nektar++ build is successful but a standalone build of the code fails due to C++ 11 support not being enabled. To address this, the fix has been rebased onto the tutorial C++ 11 fix from @ssherw (8f8ed6f9).