Simplified installation instructions

[Anders Logg (Chalmers)]

Add simplified installation instructions (as easy and non-intimidating as possible) to main download page and point to readthedocs page for details.

Make docker more visible as perhaps the recommended way of installing FEniCS.

[Martin Sandve Alnæs]

I've created a preliminary docker installation slide for fenics-course that needs to be updated for the final "fenicsproject foo" syntax.

[Martin Sandve Alnæs]

Collecting my ideas for fenicsproject script here so they don't get lost on slack:

Mapping $(pwd)/src:/fencis/home/build/src for dev-env. (see the script attached on slack for suggested solution).

Add a oneshot 'run' command

fenicsproject run image

which doesn't preserve the image between runs. Doesn't need name. (see the script attached on slack for suggested solution).

Let "/bin/bash -l" be the default command to run, but allow overriding with another commandline argument, we can allow both e.g.:

fenicsproject run stable
fenicsproject run stable "python myscript.py"

I think Jack described a way to do this on readthedocs.

[Anders Logg (Chalmers)]

Mostly fixed but couldn't figure out how to get multiple word commands (like "ls -l") to work.

[Jack Hale]

Going to resolve this and move the "ls -l" issue to its own issue.

[Jack Hale]

ls -l is in issue #33.