Factory functions should return unique pointers

[Jan Blechta]

It is natural that all factory functions (linear algebra factories, mesh generation factories, etc.) return std::unique_ptr. That can be converted to std::shared_ptr if needed then.

This might not have any noticeable performance improvements but it improves semantics of the interface. The question is how SWIG likes this change.

[Prof Garth Wells]

This would be simpler when moving to C++14 so we can use std::make_unqiue.

pybind11 (which will replace SWIG) supports unique_ptr.