Change ttim Temporal Settings: Aquifer

[Huite]

The following changes:

• stehfest_M is a misnomer, the inverse Laplace transform is the one by De Hoog, Knight, and Stokes
• time_start should be ommitted: it's a duplicate of reference date.
• then reference data should probably be renamed to start_date (or something).
• tmax can be collected from the observations and the computation times.
• tmin should probably remain.