Gitignore generated demo files

[Jan Blechta]

Generated demo files should be gitignored as a build product while not ignoring hand-written ones. There are many possible solutions.

The best one should be to only put all generated files into build dir, not into src dir. (There might be an issue for this already but I can't find it.)

[Prof Garth Wells]

Remember that demos are for users - putting them in the build dir wouldn't be very nice from this point of view.

Once all demos are rst, we can just ignore demo_*.py.

[Jan Blechta]

Agree with the second clause.

To the first one. Build artifacts in the src dirs always cause us (devs and users building from source) troubles and confusion. It seems that you're now speaking about users who get the demo by downloading dolfin repo, running cmake/scripts/generate-demo-files, not being able to locate build dir, so not even building DOLFIN itself. It seems to me that user who do not want to build DOLFIN should rather fetch the demos from either a website, or an install location (<prefix>/share/dolfin/demo?) on the machine.

[Prof Garth Wells]

I confess to not realising that install copied demos into share/ (despite the message printed at the end of a build), so I agree that we should put the generated demos in a build directory.

[Prof Garth Wells]

.gitignore updated in 661d43b.