Clarify Python installation instructions for Linux

[mstimberg]

See #290.

The changes and the rationale behind them:

• Remove the line "Install Python 3 using all of the defaults for installation", because describing how to install it is what is done in the next steps (also: "using all of the defaults" is not correct, the last answer should be "yes" to prepend Anaconda to the PATH, but the default is "no")
• Add a remark that possibly a cd Downloads or similar is necessary to find the downloaded file
• Replace "When there is a colon at the bottom of the screen press the down arrow to move down through the text." by "To move through the text, press the space key." -- the details of what you see depend on your pager (e.g. with more you see --More-- and not a colon) and the down arrow does not work with more (space works with both more and less).
• Add "Close the terminal window." in the end. If the learners leave the terminal window open, the PATH change is not applied and they can therefore not start python or the Jupyter notebook. The installer mentions this at the end, but this can be a major stumbling block so I think it is better to make this explicit in the instructions.

[wking]

e6abf07 looks good to me. As an extremely minor nit, you could add a blank line between your commit message's summary and body. For example: Clarify Python installation instructions for Linux Closes #290 instead of the current: Clarify Python installation instructions for Linux Closes #290 The blank line helps ‘git log --oneline’ and similar extract the appropriate summary.

[mstimberg]

As an extremely minor nit, you could add a blank line between your commit message's summary and body.

Sure, no problem. I rebased and force-pushed the commit with the corrected message (no other changes).

[wking]

67121cd looks good to me.