New terrain cutting models

[hgopalan]

Summary

The initial immersed forcing model marks the cell as binary (0 for outside terrain and 1 for partially/fully in terrain). In this PR we extend the method to allow for a partial cell cut. This is accomplished by assuming that the terrain can be based on the face-distant terrain height in each cell. This method is not as accurate as AMReX EB method but makes it easier to handle large wind farms since the STL size for these farms can be of the order of few GBs while the interpolation method for marking terrain is instantaneous.

The terrain_blank variable is not modified since it will also be used for ocean waves. The new variable terrain_vf stores the value of the volume fraction. This can be modified in the future if required.

Please check the type of change introduced:

• Bugfix
• [X ] Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

The existing terrain test case can be run using this method and there is no need for new unit and regression tests.
Comparison of proposed method (top) and existing (bottom)