Script generator: Create a method for organising script definitions

As a user of the script generator I do not want to be confronted with all the possible script definitions for the whole of ISIS when I create my script. Instead I would like to only see the available definitions for the instrument/technique I am using.

## Acceptance Criteria
* In the script generator I can select my instrument/technique and I will then be presented with only the definitions for that selection
* There are tests/documentation for this
* Instrument/techniques can have a help string provided for them that displays information to the user in the script generator

## Notes
* In a meeting with @Tom-Willemsen, @DominicOram, @JamesKingWork, @John-Holt-Tessella and others on 19/06/2020 it was decided that using folders inside the repository would likely be the best way to do this