[FEATURE] - Add ATS package for adaptive time stepping

[JoerivanEngelen]

After a discussion with @Manangka, we concluded that the adaptive timestepping package (ATS) is the last package required to get similar features as what users were used to in iMOD-WQ. It is easy to implement, so low-hanging fruit.

ATS can be used together with the ats_percel option in the ADV package to configure a courant number.

This should get us similar behavior as SEAWAT did.

[JoerivanEngelen]

Refinement

• Add ATS package
• Add logic to Modflow6Simulation to recognize ATS has been added to simulation and automatically link it to TDIS
• Add ATS_PERCEL setting to ADV package

[Manangka]

A description of how ATS work can be found in the mf6io document: https://water.usgs.gov/water-resources/software/MODFLOW-6/mf6io_6.6.0.pdf

There are some choses to be made for the default behavior. If you would create an ATS package and ADV package with the percel option then both packages will submit a preferred timestep size and the lowest will be picked.

If you would choose to only want the timestep restriction to be set by the ADV package then you can disable the ATS package by setting dtadj & dtfailadj to zero.

The dt0 option is used to specify the first timestep of a period. If you set it to 0 then the last timestep from the previous period is used.