Bk descriptors

[YuuuXie]

This pull request wraps up B1, B2, B3 descriptors into a Bk descriptor module. And should be merged after #12

1. When initializing the descriptor, previous setting

descriptor_settings = [n_species, n_max, l_max]
desc = B2(..., descriptor_settings)

is equivalent to current setting

K = 2 # body order. K = 1 -> B1, K = 2 -> B2, K = 3 -> B3
descriptor_settings = [n_species, K, n_max, l_max]
desc = Bk(..., descriptor_settings)

The lammps files are changed correspondingly.

2. Another feature: add atom_indices to sparse_gp.py/update_db and sparse_gp.cpp/add_training_structure to allow adding a subset of force labels, such that the Kuf matrix is smaller in the "f" dimension

Notes:

1. To add descriptors with higher body order, just need to add coefficients (l1, l2, l3 ...; m1, m2, m3, ...) to coeffs.cpp and add indices (n1 l1 m1, n2 l2 m2, ... nk lk mk) in indices.cpp.

2. The original b1/2/3.cpp are kept for unit test, but we could delete them and might find better way of doing this.

• Finish unit test for lammps
• Change compute command for variance to Bk descriptors

Todo

The lammps code can be further optimized:

• The trick as Rewrite b2 lmp #12 is not implemented
• The search of neighbor list and the computation of single bonds are duplicated for each kernel. But we can do it only once by using the maximal cutoff, n_max and l_max
• put lammps B2 back to the code