This patch implements the paper "Regularized Kelvinlets: Sculpting Brushes based on Fundamental Solutions of Elasticity" https://graphics.pixar.com/library/Kelvinlets/paper.pdf
It includes grab, biscale grab, triscale grab, scale and twist.
All Kelvinlets are accessible under the same tool. This helps to keep the code organized and it should not make any difference to the user if this proposal is implemented. https://devtalk.blender.org/t/the-tools-brush-workflow/9111/11?u=pablodp606