Feature/linear hardening

[btalamini]

We didn't have linear hardening as a standalone option. You could get it as a special case of power law or Voce by choosing the parameters in a particular way, but it's easier for users to have the linear case explicitly available.

This is a first step towards reducing code repetition. We have several J2 model implementations now, including one that only has linear hardening (but also has linear kinematic hardening). I want to have one implementation with options that are easily configurable.

There are a few other small items in this PR:

• I used the new linear hardening struct in tests that were using linear hardening behavior
• I improved the readability of an old test (behavior is unmodified)
• I switched one of the existing tests from power law to Voce, because this hardening option was not used in any test. Increases test coverage one iota.

[btalamini]

@samuelpmishLLNL , would you mind taking a look at this?