Remove Loki as a dependency, require C++11 support in library

This MR removes Loki as a dependency of Nektar++ and enables the requirement of C++11 for compiling Nektar++. In particular the following changes are made:

• Loki::Singleton is removed for our factory holders, instead using a simple static variable inside a static function, which is guaranteed to be thread-safe in C++11.
• CMake configuration is set up to require C++11
• Boost preprocessor headers have been removed in deference to variadic templates in the factory, manager and memory manager.
• .clang-format updated to C++11