ENH: FO: Lamb vector

  - If applied:
      This commit allows the user to compute:
        - the Lamb vector (https://en.wikipedia.org/wiki/Lamb_vector),
        - on-the-fly or via postProcess utility
        - for a given volVectorField (one per functionObject entry)

  - Why:
      The motivation is the literature-reported quantitative connection
      between the Lamb vector (divergence) and the spatially localised
      instantaneous fluid motions, e.g. high- and low-momentum fluid
      parcels, which possess considerable level of capacity to affect
      the rate of change of momentum, and to generate forces such as drag.

  - Verification:
      - Smooth-wall plane channel flow case (Moser et al. 1999) by
          # Curtis et al. (2008) On the Lamb vector divergence
            in Navier–Stokes flows, doi:10.1017/S0022112008002760

  - What's next:
      - The verification-show case
      - Extended code guide entry titled "Lamb vector"