Fortran interpolation

This is the Fortran interface for Legion, which supports:

1. C++ Legion with Fortran tasks
2. Fortran only Legion

Known issues:

1. When mixing C++ with Fortran, it requires to include legion_c.h, but this header file is not supposed to be exposed to users.
2. Limited by the macro support in Fortran, the macro used for generating multi-dimensions is not working in Fortran, so I manually implemented up to 3D.
3. Only tested with GNU 7 and 8.
4. Not all Legion APIs are ported into Fortran yet, will port more if needed.
5. The destructor is not automatically called by gfortran, will need to be explicitly called.