This MR replaces the current gravity term computation in LiquidFlow by using the body force vector.
These changes are for the implementation of the local assembler on arbitrarily inclined 1D or 2D element, which requires a projection of the gravitational force onto to the inclined plane, and then a mapping of the local velocity vector to the global system.
Feature description was added to the changelog
Tests covering your feature were added? Tested by the existing benchmarks.
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