Add DoF extraction function

[Psirus]

So the idea was that I could throw any Function, subfunction or expression into a function and get a numpy vector back that I could plot. Basically it does:

• Function -> f.vector()
• subfunction -> interpolate(f, fs).vector()
• expression -> project(expr).vector()

and I'm not sure anymore how useful this is. Let me know.

Edit: Reason for build failure: the behaviour seems to have changed going from 2017.2 to 2018.1 for the subfunction. Will investigate later.

[TTitscher]

Is this still relevant?

A more general approach would transform a function/subfunction/expression (optional argument is the function space/mesh?) into a Function. This can then be plotted. If you prefer the numpy.array, call .vector()[:] afterwards.

The use case for me would be:

f = XDMFFile(....)
f.write(helpers.to_function(u), t) # does basically nothing, just common interface
f.write(helpers.to_function(sqrt(u)), t)
f.write(helpers.to_function(sqrt(u), super_fine_plotting_mesh), t)