init: now Common are unordered_map
port: append index and exists functions
lib: improve the GenericCoupledModel

The GenericCoupledModel can now handle vle::Common and can be used in
recursive model hierarchy.

- BUG in port: the is_empty function is not valid.
CMakeLists: append a new unit test
CMakeLists: switch to c++1y
path: remove typo
vle: Now Echll is pure template library

Remove any cpp files in the project.