Add two hill activation function to chamber

[alexkaiser]

Problem

The "two hill" function is a popular and flexible functional form for activation in chamber models that we should include in the solver, see for example in https://link.springer.com/article/10.1007/s10439-022-03047-3. This model allows for much more flexible and physiologically realistic waveforms than a cosine activation function.

Solution

I implemented the model in a branch on my fork, https://github.com/alexkaiser/svZeroDSolver/tree/chamber_two_hill_2.

Additional context

One could include this as a switch in the existing chamber model (as I have implemented it) or make another chamber model entirely.

Code of Conduct

• I agree to follow this project's Code of Conduct and Contributing Guidelines

[alexkaiser]

@ncdorn @ktbolt What do we think of implementation and testing on this?

[ncdorn]

I think this could definitely be useful. To handle multiple activation functions I would propose the following:

• merge in Add New Heart and Valves Blocks based on the Regazzoni Heart Model #168 to create the Regazzoni Chamber block (no inductance compared to the currently implemented ChamberElastanceInductor
• open a new PR which renames RegazzoniChamber to HeartChamber (or something else) and allows for specification of different activation functions.
• the specification of the activation function could happen in the block json file. This could be some key value pair with key "activation_function" and values like "piecewise cosine" or "two hill". We would need to have a corresponding key-value pair with key "activation_function_values" and the value being a dict of the required parameters for that specific activation function.

let me know what you think @ktbolt @mrp089 we can also discuss further at the next svZeroDSolver meeting (August 21)