Move random selection of 7000 tie points to image warping to allow feature tracking with more shift vectors

As discussed in https://github.com/GFZ/arosics/issues/28, it might make sense to move the automatic sub-setting of the originally computed tie point list to the image warping instead of doing this directly when creating the GCP list. Currently, at most 7000 tie points are auto-selected when creating the GCP list within the Tie_Point_Grid module. However, moving this to a later stage would enable to use AROSICS for feature tracking with more than 7000 tie points, where the correction of image displacements is not needed at all.