Invert aquifer indices in assembly.
Enable different transmisivity for every aquifer.
Add multiple aquifer and aquitard parameters.
Multiple aquifer test, fix bud in node value getter.
Fix wrong aquifer assembly order.
Add pressure set flag in well interface.
Pressure at he top can be unknown.
Fix error computation in convergence test, clean code.

- enable convergence test again for one aquifer
- clean code, improve readability