Remove const at() method as swig will ignore it anyways.
Add norm argument to value constructor, implement member access methods, update documentation.
Correct Doxygen documentation.
Add node manipulation methods.

A set of methods have been added for node manipulation, such as retrieval of node energy and intensity, but also appending, inserting and removing of nodes or extending the node function.
Correct Doxygen documentation.
Add m_last_log_e_norm to cache.
Complete value constructor, add inline methods.
Add m_last_log_e_norm to cache.
Add evaluation cache, complete value constructor, add member access methods.
Remove const from eval_gradients().
Implement value() constructor, remove const from eval_gradients(), add parameter methods.
Rename norm() to prefactor().
Add GModelSpectralPlaw unit test.
Add GModelSpectralPlaw2 unit test.
Rename norm to prefactor.
Rename ecut to cutoff.
Add GModelSpectralExpPlaw unit test.
Rename norm to value.
Add GModelSpectralConst unit test.
Rename norm to prefactor