Small modification in notebooks

[afoix]

Modification in notebook 1

• Add some python idioms
• Change the name of ordered() function to sorted_by_distance()
• Add comments and description in the skeletonise and sorted_by_distance() functions

Modification in notebook 2A

• There is a typo in 2.6 that makes the don’t run. It’s an extra ] that I’ve remove it.
• Add comments in 0.1 in the code block of the number of points. Explanation about the dimensions.
• First code block of 1.3 add explanation about imaginary numbers and its format (Cartesian form)
• 1.4 second block, explanation of the exponential form of the imaginary.
• In the regular notebook 2A in the first code block of 3.2 there his a dtype=np.complex, this does not work, it need to be dtype=complex