Adding an API documentation for new forks of the PINN approach

[nih23]

• [x] add setter for HPM module in PINN interface (setHPMnetwork)
• [x] add branching in pde loss in PINN interface => go either into analytical PDE loss or HPM PDE loss
• [ ] rename "initial_loss" into "interpolation_loss"
• [ ] add train method to PINN interface
• [ ] add interface for models within PINN interface => e.g. one/two method should provide additional loss terms (if necessary) for interpolation and PDE learning

[StillerPatrick]

Is solved by #7