Resolve "Modify RichardsSimulation to have the base class #94."

What does this MR do?

Modify RichardsSimulation to have the base class #94 (closed).

• This MR depends on #90 (closed) since the interface is not complete without separation between adaptivity and marking the grid.
• All the constants on BaseTraits are removed so that RichardsSimulation is using policies introduced in #94 (closed). Nevertheless, a static check is still done for adaptivity in order to allow only UGgrids to be adapted.
• The suggest_timestep() method is now also implemented in TimeController
• Method run() is overloaded temporally while !82 (merged) is finished.

Is there something that needs to be double checked?

Can this MR be accepted?

• Implemented
• Pipeline passing
• Added entry to CHANGELOG.md
• !87 (merged) accepted
• !91 (merged) accepted

Related issues

Closes #95 (closed), #90 (closed)