Prescribed deformation gradient

[sswan]

When you prescribe the deformation gradient, it's immediately recast in terms of strain (defined by the current kappa) and then that strain is linearly interpolated over the step. At each point along the interpolation, the strain is converted to deformation gradient.

This gives a non-linear behavior when you look at the deformation gradient components in the output. When you would expect a nice straight line over the step, you get a curve.

See the test in /tests/swan/test_defgrad_basic.py.