Make "Getting ready" easier

[olexandr-konovalov]

At the moment "Getting ready" requires not only just to start GAP, but also to create a directory and make it a current directory for GAP.

This requires:

• some actions with the file system, which may not be clear to the reader (especially when there were no UNIX shell lesson preceding GAP lesson)
• calling functions from the GAP command line way before we explained how to use it (case-sensitivity, tab completion, commands history are not yet explained, so that may be very painful and time consuming.

Hence we should change the "Getting Ready" part so that it is enough just to start GAP at any location for the first episode. In particular:

• it should be explained how to start it in different operating system in more details ("Run bin/gap.sh script to start GAP" in GAP installation instructions is not clear to those who had never used the terminal). It should be explained how to check that installation has been successful.

This will allow more smooth start of the lesson, without losing pace to solve technical problems, and making part of the audience waiting for another part of the audience.

We will need file operations later - we can explain paths and directories in the episode on functions before introducing the Read command, and then introduce LogTo in the episode on regression tests (agree with @ChrisJefferson in #14).

Before creating a file with the GAP code and trying to load it using Read, the reader should be able to create a directory and a file with the GAP code in that directory, understand the concept of the current directory, and know how specify path to a file dependently on the operating system.

[olexandr-konovalov]

The way to start GAP changed - no bin/gap.sh any more. That should be updated, possibly more.