Revise incar.py with two new classes *Param* and *Params*, still comp…

[hsulab]

…atible with previous usage, and make INCAR writing simple and extendable with VASP INCAR Parameters DataBase (vasp_para_db.py).

[PytLab]

Hi, thanks for your contribution. As I get an urgent issue to address recently, will review the change once the issue is resolved. :)

[codecov-commenter]

Codecov Report

Merging #14 into master will decrease coverage by 0.14%. The diff coverage is 17.44%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
- Coverage   21.73%   21.58%   -0.15%     
==========================================
  Files          21       22       +1     
  Lines        1537     1705     +168     
==========================================
+ Hits          334      368      +34     
- Misses       1203     1337     +134     

Impacted Files	Coverage Δ
vaspy/incar.py	15.92% <15.13%> (+2.16%)	:arrow_up:
vaspy/vasp_para_db.py	100.00% <100.00%> (ø)
vaspy/__init__.py	74.07% <0.00%> (-3.71%)	:arrow_down:

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