Added a function in Pack to stack compounds and build a single layer

[chemicalfiend]

This stack function helps initialize a system where a bunch of chains are stacked in a single layer, one on top of the other at a given separation.

[codecov[bot]]

Codecov Report

Merging #92 (6f32629) into master (d3cb512) will decrease coverage by 0.36%. The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
- Coverage   98.44%   98.07%   -0.37%     
==========================================
  Files           6        6              
  Lines         193      208      +15     
==========================================
+ Hits          190      204      +14     
- Misses          3        4       +1     

Impacted Files	Coverage Δ
planckton/init.py	98.83% <93.33%> (-1.17%)	:arrow_down:

[chemicalfiend]

Made the changs requested.

[jennyfothergill]

@chrisjonesBSU pre-commit will make the changes if you have it installed

https://pre-commit.com/#install