Clean up C++ API.

[amcadmus]

Changes

• Switch to new interface of neighbor list
• Add unittests
• Clean up the code. remove out-dated interfaces and implementations

[codecov-io]

Codecov Report

Merging #409 (0fa68a1) into api (3c268cb) will decrease coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              api     #409      +/-   ##
==========================================
- Coverage   60.13%   60.02%   -0.12%     
==========================================
  Files          65       65              
  Lines        5163     5148      -15     
==========================================
- Hits         3105     3090      -15     
  Misses       2058     2058              

Impacted Files	Coverage Δ
deepmd/entrypoints/freeze.py	19.56% <ø> (ø)
deepmd/infer/data_modifier.py	93.44% <100.00%> (+0.02%)	⬆️
source/op/_tabulate_grad.py	57.14% <0.00%> (-9.53%)	⬇️
source/op/_gelu.py	71.42% <0.00%> (-6.35%)	⬇️
deepmd/loss/tensor.py	55.76% <0.00%> (-0.84%)	⬇️
deepmd/loss/ener.py	7.31% <0.00%> (-0.45%)	⬇️
source/op/_prod_force_grad.py	100.00% <0.00%> (ø)
source/op/_prod_virial_grad.py	100.00% <0.00%> (ø)
source/op/_soft_min_force_grad.py	100.00% <0.00%> (ø)
source/op/_prod_force_se_a_grad.py	100.00% <0.00%> (ø)
... and 4 more

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