Prepares P2P1SurrogateTaylorhoodStokesOperator for 3D

In the P2P1BlendingTaylorhoodStokesOperator we replace

 - P2ToP1ConstantDivzOperator    div_z;
 - P1ToP2ConstantDivTzOperator   divT_z;

by

 - P2ToP1BlendingDivzOperator    div_z;
 - P1ToP2BlendingDivTzOperator   divT_z;

For this the blending versions get typedefed in P1ToP2SurrogateOperator
resp. P2ToP1SurrogateOperator.

Note that while the forms are already implemented, the P2P1 operator
will not work, as the variable operators themselves are currently only
available in 2D in the master.

However, IMHO this way is cleaner, as we have no hidden "wrong" imple-
mentation, but get a nice and clean run-time abort.
23 jobs for mohr/minor_fix in 158 minutes and 10 seconds (queued for 1 minute and 41 seconds)
Name Stage Failure
failed
benchmark_clang8 Test
  File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 404, in _grab_next_args
for seg in self._plot_args(this, kwargs):
File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 384, in _plot_args
x, y = self._xy_from_xy(x, y)
File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 243, in _xy_from_xy
"have shapes {} and {}".format(x.shape, y.shape))
ValueError: x and y must have same first dimension, but have shapes (14,) and (0,)
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
benchmark_gcc9 Test
  File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 225, in __call__
yield from self._plot_args(this, kwargs)
File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 391, in _plot_args
x, y = self._xy_from_xy(x, y)
File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 269, in _xy_from_xy
raise ValueError("x and y must have same first dimension, but "
ValueError: x and y must have same first dimension, but have shapes (14,) and (0,)
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
benchmark_intel19 Test
  File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 404, in _grab_next_args
for seg in self._plot_args(this, kwargs):
File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 384, in _plot_args
x, y = self._xy_from_xy(x, y)
File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 243, in _xy_from_xy
"have shapes {} and {}".format(x.shape, y.shape))
ValueError: x and y must have same first dimension, but have shapes (14,) and (0,)
Cleaning up file based variables
ERROR: Job failed: exit code 1