Changes

Fix homotopy option to ensure correct initialization if homotopy is used

Removed homotopy from equation for port pressure loss: having no
hydraulic resistance caused a singularity at initialization

Used proper type for temperature difference to avoid issues with K<->degC conversion

Added option to set simplified homotopy model in case the check valve of the pump is closed at initialization
Adapted the PumpingSystem example accordingly

Use NoHomotopy as default and document.

Generalize homotopy for checkValve to have three options: Open, Closed, and NoHomotopy (i.e. unknown).
The description of the enumeration is rudimentary.

Set homotopy flags in example models to facilitate the convergence of the initial solver

Added reference to homotopy parameter in the documentation of the examples