rework interface to enforce Unitful conventions

[jrdegreeff]

Open questions:

• what is the dimension of virial_stress? (seems to be units of energy to me, but I'm not sure if that's right)
• what are the dimensions of the parameters in bm, coulomb, morse, and zbl potentials?
• why is ϵ0 a parameter instead of a constant?

[codecov-commenter]

Codecov Report

Merging #29 (f375899) into main (52f2e55) will increase coverage by 5.10%. The diff coverage is 46.66%.

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
+ Coverage   41.32%   46.42%   +5.10%     
==========================================
  Files          10       10              
  Lines         196      168      -28     
==========================================
- Hits           81       78       -3     
+ Misses        115       90      -25     

Impacted Files	Coverage Δ
src/PotentialTypes/EmpiricalPotentials/bm.jl	0.00% <0.00%> (ø)
src/PotentialTypes/EmpiricalPotentials/coulomb.jl	0.00% <0.00%> (ø)
src/PotentialTypes/EmpiricalPotentials/morse.jl	0.00% <0.00%> (ø)
src/PotentialTypes/EmpiricalPotentials/zbl.jl	0.00% <0.00%> (ø)
...PotentialTypes/MixedPotentials/mixed_potentials.jl	90.47% <90.90%> (ø)
src/NeighborList/nnlist.jl	96.29% <100.00%> (+2.96%)	:arrow_up:
...lTypes/EmpiricalPotentials/empirical_potentials.jl	96.15% <100.00%> (ø)
src/PotentialTypes/EmpiricalPotentials/lj.jl	38.46% <100.00%> (+12.37%)	:arrow_up:

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