Fixes to several small oversights

This MR includes fixes for several small oversights:

• Computation of P2 diagonal with varying coefficient in 2D. Edge DoF indices must be adjusted between FEniCS and HyTeG ordering. However, they used to be shuffled twice (in lines 600-602 and again in lines 609-611). Details are explained below.
• Adding single elements to a PETSc sparse matrix proxy added an unitialized value if real_t and PETScReal were different types.
• The P2ConstantOperator used to segfault when passed a blending form. Now, the form initializes the geometry map to the identity.
• Vertex, edge and volume DoF functions now use Kahan summation in the sumLocal operation. This fixes a test failure (icx, single precision, #236) caused by inaccurate pressure projection.
• Also includes some improvements to and a new test for the P2 diagonal implemented in DiagonalNonConstantOperatorTest.