generalisation of getImagettes for labels to avoid redundant code, some changes to input of spam-pixelSearchPropagate

[skip-ci] safety in visual clients file name guessing, input parameters for ereg and sketch of new DIC practice doc

grid example now complete in tests (spam-pixelSearch + spam-ldic + spam-regularStrain). Registration checked in 2D

[skip-ci] spam-ldic now working, with no pixelSearch and -pfd option; spam-pixelSearch only missing -skp

[skip-ci] grid pixel search fully tested, starting on label mode, some trouble re-reading previous result even without registration and zero search range...

new pixel search that checks all combinations and returns a position in im2 (simpler but less useful)

This reverts commit 750dfd34