Missing forward declaration.
Correct header.
New API is expecting a qoi index.
Constructor now takes the qoi name. All other stuff moved to init.
Rename get_qoi to get_qoi_value as it's more descriptive.

And we'll need a get_qoi method that will return a const reference
to the QoI object.
Fixes for get_qoi method.
Updated echo_qoi to use CompositeQoI`