Draft: Add examples (WIP)

This MR adds examples of how to use memory manager in C++/C/Fortran.

• C++ simple example
• C simple example
• Fortran simple example
• C++ use external library example (ginkgo)
• Compare get_var using index/var descriptor
• C++ GPU example
• C++ GPU + OpenACC kernels example
• Fortran use C++ function example
• Use metadata example