Explain sympy.expand( ), fix more notation issues

Language remained giving the idea that expand() and simplify() are
almost interchangeable. This is not correct: they have different
purposes.
Add link to documentation for solve(), and explain a bit more how it
works.
Merge branch 'wave_dev' of https://github.com/numerical-mooc/numerical-mooc into wave_dev

Conflicts:
	lessons/03_wave/03_03_aBetterModel.ipynb