Implement a better way to handle the unassigned fibers

During the work of INSTRM-2336 we found that the most of the cobras with very big step number are because they are too close/far way from center. The case of cobras too close to the centers are unassigned fibers. We set the locations of unassigned fibers to be

targets[isNan] = self.cc.calibModel.centers[isNan] + (0.04 + 0.04j)

We need a better way to handle those unassigned fibers.