[Lesson 01] Defined W as the weight in the text.

The variable `W` was not defined in the text.
Merge branch 'phugoid_solution' of https://github.com/numerical-mooc/numerical-mooc into phugoid_solution

Conflicts:
	lessons/01_phugoid/01_01_Phugoid_Theory.ipynb
[phugoid script] Add docstrings instead of comments.

Docstrings replaced comments and define the arguments of the function.

Increased the size of title and labels in the plot.
Merge remote-tracking branch 'origin/phugoid_solution' into phugoid_solution
Resolved conflict with style of notebook not cleared.