Null load case doesn't allow deformation

[gcapes]

We need a zero normal stress load case for this to produce a load file like this:

loadstep:
- boundary_conditions:
    mechanical:
      F:
      - [x, 0, 0]
      - [0, x, 0]
      - [0, 0, x]
      P:
      - [0, x, x]
      - [x, 0, x]
      - [x, x, 0]