fixed notebook bugs for derivatives, plotplanck and stefan_boltzman_answer
one more typo in stefan_boltzman_answer
tweak plotplanck notebook
fix typo in derivitives notebook
wiens law solution
fix typo
solutions for day 5
change name of notebook
Revert "day 5 commits"

This reverts commit fcb9c9e25ffb0f68da18f63d4258c919cf46c023.

Conflicts:
	notebooks/derivatives.ipynb
	notebooks/plotplanck.ipynb
	notebooks/stefan_boltzman_answer.ipynb