improvements for lesson 07

[m-jahn]

• this lesson is far too long compared to the others
• remove gzip, or at least the erroring gzip example, no obvious point for that
• consider removing argparse, which is for standalone python programs, not so relevant for coding beginners
• focus should be on the most useful libraries, which is pandas, and, to much lower degree, numpy
• remove section on numpy linked arrays, not relevant to show, can just mention in 1 line
• shorten section on numpy shapes considerably
• remove section on timing or reduce to just 2-way comparison
• break up code chunks for all examples, currently they are often written as 1 block which is not really instructive for participants who should run examples in a sequential order
• remove pandas part where data is not loaded with pandas
• shorten mark down content to most important points