Ase interface

[YuuuXie]

1. add some docs
2. added test_2_body and test_3_body to test_mgp.py

[codecov-io]

Codecov Report

Merging #121 into master will increase coverage by 2.95%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   56.23%   59.19%   +2.95%     
==========================================
  Files          31       31              
  Lines        5002     4749     -253     
==========================================
- Hits         2813     2811       -2     
+ Misses       2189     1938     -251

Impacted Files	Coverage Δ
flare/mgp/cubic_splines_numba.py	8.44% <ø> (+2.56%)	⬆️
flare/dft_interface/__init__.py	75% <100%> (ø)	⬆️
flare/mgp/mgp.py	98.61% <0%> (+0.27%)	⬆️

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