Allow loading of complete NeuroML models containing <include>

[pgleeson]

NeuroML files can include external files to avoid repetition of elements, e.g. <include href="hhneuron.cell.nml"/> in https://github.com/openworm/hodgkin_huxley_tutorial/blob/master/Tutorial2/NeuroML2/HHTutorial.net.nml

The C++ API should have a helper function to support the loading of the full model, e.g. as in the Python case: https://github.com/NeuralEnsemble/libNeuroML/blob/development/neuroml/loaders.py#L230

Then a summary can be printed for the full model, using this #8

@tuigimdochas