[RANS] Make calculation of velocity gradients and wall functions more general

Inspired by #532 (closed).

When considering a domain that features a pointy tip like illustrated below, it might be the case that the cell at the tip does not have any vertical or horizontal neighbors. Right now, in this case, the gradient in this case will be set to 0 (!1130 (merged)).

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The should be further generalized to such that all wall functions, gradients, and functions for the boundaries are created in a separate class, rather than in each problem file.