Resolve "[Transport] Local Operator for finite volume scheme"

What does this MR do?

Implements the Local Operator the transport equation for the Finite Volume method.

Since a Local Operators can't be tested by itself, there is no test for this. In this sense, you can check that it does not have syntax errors and can work as a PDELab local operator since this branch is merged into !93 (merged). On the other hand, the mathematics cannot be checked until I provide a test for !95 (closed) or !96 (merged). Therefore, fixes of bugs on this operator have to be done on the fly while I am adding functionalities to the transport simulation.

Can this MR be accepted?

• Change the name of the local operator file for richards to richards_operator_DG.hh
• Implement the alpha_volume() method.
• Implement the jacobian_volume() method.
• Implement the alpha_skeleton() method.
• Implement the jacobian_skeleton() method.
• Implement the alpha_boundary() method.
• Implement the jacobian_boundary() method.
• Implement the lambda_volume() method.
• Implement the suggestTimestep() method.
• Added entry to CHANGELOG.md

Related issues

Closes #73 (closed)