add test for zap scan type

zap scan = 1 fast axis (continuous) + 1 slow axis (step by step)

simulates meshgrid points by emitting Ypos = [ypos,]*Nx with Nx number of points along x axis

displayed as 2D scatter map in Flint

=> add a test to simulates a zap scan to test the channels management by writer/pymca (groups) and Flint (data alignment)

see id21/id21#2