Cluster pbc

The cluster function of minimal_box was made to adjust the unit cell in all directions to get the desired grid spacing, h. Initially, we wanted to expand this to work with pbc, where the box in the periodic directions is conserved, and the desired h becomes the average of the h in the periodic directions. While doing this, we concluded that the Cluster object is not needed other than the minimal_box function, which can be a stand-alone function, here adjust_cell, as the ase Atoms object is sufficient. Therefore, we replaced the minimal_box function with the adjust cell from the test and documentation files. The adjust_cell function works as the old minimal box function but also takes the pbc into consideration, as we wanted. The minimal_box function in the Cluster object, as of now returns the adjust_cell but we would like to face out the Cluster.