change composite vep to log iteration

[bobmyhill]

This PR changes the iterative scheme for the composite viscoplastic rheology to use the log stress and strain rate. This should be a more linear system and therefore be more efficient.

[bobmyhill]

/rebuild

[bobmyhill]

Hi @naliboff, after I fix the bug, this will do the same thing as @lhy11009's change to Peierls. It will be marginally more expensive, but not sufficient to need a further change

[lhy11009]

@bobmyhill, I agree with @bobmyhill , this is really neat solution.

[bobmyhill]

Ok, the problem here is with the (damped) Drucker-Prager, where dln(strain_rate) / dln(stress) = stress / (stress - yield_stress). At the yield stress, this value is undefined. For the undamped scheme, the strain rate is also non-unique above the yield stress, making the problem worse.

With a damped plastic element there is always a unique stress for any given strain rate, but a naive Newton scheme without line search is going to struggle to find it. With an undamped plastic element an iterative scheme is not required; if the plastic element is active then it accommodates all the strain - so this would be easy enough to implement.

Not sure what the best solution is to this problem. Line search (e.g. using SUNDIALS) would certainly help. Maybe there's another option.

FYI: @lhy11009, @naliboff