Idea: fixed-index mapping about grits HOT 3 CLOSED

    mapping : dict or path, default None
        Either a dictionary or path to a json file of a dictionary. Dictionary
        keys contain desired bead name and SMARTS string specification of that
        bead and values containing list of tuples of atom indices::

            mapping = {"_B...c1sccc1": [(0, 4, 3, 2, 1), ...]}

>>> atoms_per_mol = 20
>>> num_mols = 30
>>> mapping = {"A": []}
>>> for n in range(num_mols):
...     mapping["A"].append(np.array([10, 11, 12]) + (n * atoms_per_mol))

>>> mapping
{'A': [array([10, 11, 12]), array([30, 31, 32]), array([50, 51, 52]), array([70, 71, 72]), array([90, 91, 92]), array([110, 111, 112]), array([130, 131, 132]), array([150, 151, 152]), array([170, 171, 172]), array([190, 191, 192]), array([210, 211, 212]), array([230, 231, 232]), array([250, 251, 252]), array([270, 271, 272]), array([290, 291, 292]), array([310, 311, 312]), array([330, 331, 332]), array([350, 351, 352]), array([370, 371, 372]), array([390, 391, 392]), array([410, 411, 412]), array([430, 431, 432]), array([450, 451, 452]), array([470, 471, 472]), array([490, 491, 492]), array([510, 511, 512]), array([530, 531, 532]), array([550, 551, 552]), array([570, 571, 572]), array([590, 591, 592])]}