Get rid of demo_main.py

[Martin Sandve Alnæs]

Make demos self-contained and runnable directly:

./demo_foobar.py # using default choice of sceme and fields

We still need to be able to easily run the same problem with different schemes and parameters, so I want to be able to write:

./demo_foobar.py --scheme=ipcs --scheme.dt=.2e-3 --problem.mu=0.0035

For 1.3 I suggest we stick with defining fields in the demo.py file and figure out something better later.

[Martin Sandve Alnæs]

Issue #14 was marked as a duplicate of this issue.

[Martin Sandve Alnæs]

To summarize our discussion, we will:

• Move all construction of scheme and fields and solver to main in each demo file
• At a later point implement a cmdline tool to run a problem with a different solver