Improved contraction pressure

[MaxBalmus]

Right now it is possible to build models where you can have lower active contraction stiffness than the passive one. This means that when the hear contracts, and the pressure should build up to exceed the downstream values, this opposite actually happens.

A simple fix is to move away from the current mixed model approach, where the total pressure is weighted sum of active and passive components, to a version where:

• the is always a baseline passive pressure
• the active component is an additional term modulate by the active function
  What does this mean in practice this would mean the interpretation of some parameters changes, particularly when it comes to E_act, which could be interpreted as ESPVR.