Add experimental moving domains

Implement facilitates to move the domain depending on primary variables. All primary variables are then conserved with respect to the moved domain instead of the coordinates of the grid. This is achieved by a new input parameter on the model section:

[model.domain.deformation]
# use primary variables to express how domains move (e.g. dx and dy)
x.expression = dx
y.expression = dy

[model.scalar_field.dx]
# primary variable named dx, solved implicitly and monolithically with the rest of variables

[model.scalar_field.dy]
# primary variable named dy, solved implicitly and monolithically with the rest of variables

The resulting vtk file is not "moved" with respect to the domain. Instead, the user is expected to interpret the deformation by his own (e.g. using "Warp by Vector" on preview).