Grid Points Per Dimension should be None, when system=homogeneous and partition=manifold

In the case of homogeneous systems with partition_method = manifold, the parameter 'Grid Points Per Dimension' should be accepted by the parser only as 'None', because it is not used anywhere in the algorithm (so, do not accept e.g. [3,3]; or at least throw a warning that this is not used, as the system is homogeneous and the partition method is manifold). In view of this, in nl_homogeneous.txt we should change Grid Points Per Dimension to be None.