FEAT: Add code execution testing through Travis CI

[mmcky]

This PR will enable code execution checks for PR's. The travis instance will:

1. compile changed RST files through sphinx
2. run an execution check to make sure the notebook executes without error.

a green travis tick indicates the notebook executes without errors.

In the future travis can be expanded to include automatic deployment of html. See Issue #543

[mmcky]

• migrate script components to bash script and include the conditional case of no changed files

[mmcky]

• @mmcky to check behaviour of travis once this is merged to master

[mmcky]

@jstac this is ready -- but let's wait on merging this until next week after Tom has tried out jupinx.

[mmcky]

@jstac this should be independent of jupinx so happy for you to merge when you wish to try out execution testing on all PRs - that change any RST files. Scratch that -- this modifies Makefile so that is why I wrote the above. Let's leave this just in case for the week.

[mmcky]

@jstac this is now ready for merge when you're ready to give CI a try on execution testing

[mmcky]

@jstac this no longer adjust the Makefile so I will go ahead and merge this to give it a trial run for any new incoming PRs. Let me know if it's a hassle and we can remove it.