Make dev-env map src directory somehow

[Martin Sandve Alnæs]

Running a dev-env environment should map /fencis/home/build/src to some host directory, if the intention is to edit sources on the host and using the build scripts in the images.

[Jack Hale]

fenicsproject script now shares $HOME/fenics/build/src out to the host if you set export FENICS_SRC_DIR=$(pwd)/src before running fenicsproject create.

In addition the compiled objects in build/ are now mounted in a Docker volume. This allows the build/ directory to be read-only mounted in another dev-env-based container and shared. This final bit still needs to be implemented in the fenicsproject script.