Fix for Lattice box and Tensile simulation. Add more unit tests

[chrisjonesBSU]

While adding unit tests for the welding and tensile simulations I ran into a couple issues.

1. The box wasn't being set in a way that avoids overlapping particles along the boundaries. Now, the box lengths are longer in each direction by one lattice constant, and by a small amount (3 angstrom) in the z direction.

2. I fixed a couple issues with the tensile simulation class.

[marjanalbooyeh]

LGTM. Thank you!

[codecov]

Codecov Report

Merging #29 (9957b9b) into main (c5dd6e7) will increase coverage by 4.52%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
+ Coverage   84.65%   89.17%   +4.52%     
==========================================
  Files          18       18              
  Lines        1303     1303              
==========================================
+ Hits         1103     1162      +59     
+ Misses        200      141      -59     

Files Changed	Coverage Δ
hoomd_organics/base/system.py	81.92% <100.00%> (+0.22%)	⬆️
hoomd_organics/library/simulations/tensile.py	100.00% <100.00%> (+80.00%)	⬆️

... and 2 files with indirect coverage changes