Resolve "Implement an abstract base class for initial conditions"

What does this MR do?

Implement a new scheme for initial conditions based on the proposal in https://ts-gitlab.iup.uni-heidelberg.de/dorie/dorie/issues/131#note_20535:

• Implements abstract base classes for initial conditions and their transformations
• Adds factories for creating generic and model-specific initial conditions

Is there something that needs to be double checked?

No.

Can this MR be accepted?

• #131 (closed) Implement an abstract base class for initial conditions: InitialCondition.
• #131 (closed) Implement a initial shape selector based on inifile information. InitialConditionFactory
• #131 (closed) Use InitialCondition in Richards/TransportSimulation.
• Added unit test for initial condition input
• Added docstrings and doxygen module docs
• Added user doc page
• Pipeline passing
• Added entry to CHANGELOG.md

Related issues

Meta-issue: #122 (closed)

Closes #131 (closed)