Fix boundary conditions and initial condition in the MRF

Issue/feature addressed

The current framework to set boundary conditions in the incompressible solver is very complex. In the current code, all boundaries are treated in one function. It is very hard to add a new type of boundary condition. I wish to reorganise the boundary condition class (Extrapolate) and to treat each boundary separately. In the new framework, all velocity components and pressure in each piece of boundary are set together. The different boundary types are generated using a factory pattern to make it flexible. In the future, the partial-slip boundary condition and the turbulent wall model can also benefit from this framework.

Also, fix this issue. close #335

Proposed solution

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Implementation

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Tests

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Checklist

• Functions and classes, or changes to them, are documented.
• User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).