Fix `Lattice` system class for coarse-grained molecules

[chrisjonesBSU]

This fixes #128 by adding a try/except statement to Molecule._align_backbones_z_axis in the case that the particles don't have elements.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.41%. Comparing base (7f85dc3) to head (92263ca). Report is 1 commits behind head on main.

:exclamation: Current head 92263ca differs from pull request most recent head 1246777. Consider uploading reports for the commit 1246777 to get more accurate results

Additional details and impacted files

[](https://app.codecov.io/gh/cmelab/flowerMD/pull/129?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cmelab) ```diff @@ Coverage Diff @@ ## main #129 +/- ## ========================================== + Coverage 94.40% 94.41% +0.01% ========================================== Files 25 25 Lines 1840 1845 +5 ========================================== + Hits 1737 1742 +5 Misses 103 103 ``` | [Files](https://app.codecov.io/gh/cmelab/flowerMD/pull/129?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cmelab) | Coverage Δ | | |---|---|---| | [flowermd/base/molecule.py](https://app.codecov.io/gh/cmelab/flowerMD/pull/129?src=pr&el=tree&filepath=flowermd%2Fbase%2Fmolecule.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cmelab#diff-Zmxvd2VybWQvYmFzZS9tb2xlY3VsZS5weQ==) | `96.83% <100.00%> (+0.06%)` | :arrow_up: | | [flowermd/base/system.py](https://app.codecov.io/gh/cmelab/flowerMD/pull/129?src=pr&el=tree&filepath=flowermd%2Fbase%2Fsystem.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cmelab#diff-Zmxvd2VybWQvYmFzZS9zeXN0ZW0ucHk=) | `92.00% <ø> (ø)` | |