Add Land-Niederer Active Stress Model

Problem

The Land-Niederer (2017) model is a phenomenological description of cardiomyocyte contraction. Like the Regazzoni-Quarteroni model that was merged from PR #596, it takes in inputs of Calcium (from the EP ionic model), fiber stretch, and fiber stretch rate and returns a scalar active tension value that can feed into the PK stress of the mechanics solver. Adding more contraction models will be useful for future numerical studies and provide more model choices to users.

Solution

I have started working on a branch that has implemented this contraction model. But a couple changes need to be made before I submit a PR: unit tests on the contraction ODE, determining an appropriate reference solution for integration test, and updates from latest commits to the main repo.

Additional context

Future work: we are planning to add a Regazzoni-style stabilization that is consistent with the gen-alpha time-stepping scheme used in the solver. Since the stabilization term for Land-Niederer was derived in this paper, it makes sense to use this to test out this stabilization.

Code of Conduct

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