Support "tolerance" keyword

[funsim]

AndersLogg proposed to expose the built-in adaptivity functionality of FEniCS.

Synatx could be:

@fenicsbot Solve Poisson in domain UnitSquare with f=sin(2*pi*x[0])*sin(2*pi*x[1]) and tolerance=0.001

[funsim]

Commit d49d10f implements this for the Poisson solver