Clarify the _make_full_featurelist and uniform_gas scale in version 1

[Nanco-L]

Answer to the question from @hswoo369.

1. _make_full_featurelist
   https://github.com/Nanco-L/SIMPLE-NN/blob/8acf48e90ecc072030417c4586d1362f145e55e1/simple_nn/utils/__init__.py#L63
   The difference between use_idx == True and False is the existence of idx_list.
   But in the case of use_idx == True, idx_list is not generated. This is the missing part.
   In the case of use_idx == False, one can find the value of idx_list.
   idx_list[jtem].append([i]*tmp_data['N'][jtem])
i is the arbitrary structure index (1 for the 1st pickle file, 2 for the 2nd pickle file, etc)
   But in current status of refactored code, this part is not needed.

2. uniform_gas scale
   I do not exactly know about the uniform gas scale method.
   But in the code, symmetry function parameters are used.
   Thus, we keep this part as we talked about today (210512) and change it if generalization is required.