fenicsproject script: implement option for sharing cache

[Jan Blechta]

We could have

$fenicsproject --shared-cache run <experimental-image>
WARNING: You're using shared JIT cache. Hic sunt leones!

Similarly

fenicsproject --cache=cache-crazy-ffc-changes run <experimental-image>

[Prof Garth Wells]

I'm not convinced that this is a good idea. Unless someone really wants it, I suggest closing this issue. It can be reopened in the future if there is demand.

[Jan Blechta]

Whatever.

[Prof Garth Wells]

Re-open later if there is demand.