Make plot_2d_decomposition use Geometry instead of TensorFemSpace #532

The function plot_2d_decomposition does not have to be a method of TensorFemSpace. It needs the following information:

• A DomainDecomposition object which provides:
  • The MPI Cartesian communicator on the single patch
  • The subdomain local to the process through the properties starts and ends
  • All subdomains through the properties global_element_starts and global_element_ends
• The breakpoints along the axes of the logical domain (eta1, eta2)
• The mapping from the logical coordinates (eta1, eta2) to the physical coordinates (x, y)

After PRs #527 and #518 a Geometry instance will contain all the necessary information. In particular, it will provide the breakpoints through the grid property. Hence, it can be passed to this plotting function as an argument instead of the parameters self : TensorFemSpace and mapping : BasicCallableMapping. It is also possible to make this function a method of Geometry.