Add unit.py to manage unit conversions

[Ericwang6]

1. Add unit.py to manage unit conversions
2. Fix bug of long forces in sqm.out
3. Fix bug of units in parsing forces in sqm.out

[codecov-commenter]

Codecov Report

Merging #202 (e05807d) into devel (3c595ca) will increase coverage by 0.12%.
The diff coverage is 92.17%.

@@            Coverage Diff             @@
##            devel     #202      +/-   ##
==========================================
+ Coverage   81.54%   81.66%   +0.12%     
==========================================
  Files          54       55       +1     
  Lines        4639     4719      +80     
==========================================
+ Hits         3783     3854      +71     
- Misses        856      865       +9     

Impacted Files	Coverage Δ
dpdata/unit.py	88.15% <88.15%> (ø)
dpdata/abacus/scf.py	87.07% <100.00%> (+0.08%)	⬆️
dpdata/amber/md.py	97.53% <100.00%> (+0.06%)	⬆️
dpdata/amber/sqm.py	98.82% <100.00%> (+0.04%)	⬆️
dpdata/cp2k/output.py	97.70% <100.00%> (+<0.01%)	⬆️
dpdata/gaussian/log.py	100.00% <100.00%> (ø)
dpdata/gromacs/gro.py	98.76% <100.00%> (-0.02%)	⬇️
dpdata/pwmat/atomconfig.py	97.59% <100.00%> (-0.03%)	⬇️
dpdata/pwmat/movement.py	77.77% <100.00%> (ø)
dpdata/qe/traj.py	85.21% <100.00%> (-0.11%)	⬇️
... and 1 more

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[Ericwang6]

@amcadmus @njzjz I've added a module called unit.py to manage all unit conversions in dpdata

[amcadmus]

great implementation, minor change required, thanks!

[njzjz]

Approve after fixing docstring