refactor hybrid_a_star codes and clean up heuristic function names

[AtsushiSakai]

fix #363

[codecov]

Codecov Report

Merging #371 into master will increase coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
+ Coverage   89.53%   89.77%   +0.23%     
==========================================
  Files         121      121              
  Lines        8660     8623      -37     
==========================================
- Hits         7754     7741      -13     
+ Misses        906      882      -24     

Impacted Files	Coverage Δ
...nning/HybridAStar/dynamic_programming_heuristic.py	86.20% <100.00%> (ø)
PathPlanning/HybridAStar/hybrid_a_star.py	89.37% <100.00%> (ø)

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