Add mol2 files for PEEK and PEKK

[chrisjonesBSU]

I've noticed some issues with using the SMILES stricks to generate the monomers each time. I'm not exactly sure if it was caused by the change to RDkit in mbuild, of if the SMILES strings need tweaking, but I'm adding mol2 files for PEKK and PEEK (para and meta) as well as updated .JSON files that work with the .mol2 files. This should be more reproducible.

[codecov[bot]]

Codecov Report

Merging #55 (763b7b5) into master (d997cfe) will increase coverage by 0.80%. The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   93.95%   94.75%   +0.80%     
==========================================
  Files           6        6              
  Lines         480      477       -3     
==========================================
+ Hits          451      452       +1     
+ Misses         29       25       -4     

Impacted Files	Coverage Δ
uli_init/system.py	94.16% <60.00%> (+1.47%)	:arrow_up:

[chrisjonesBSU]

I tested this out with peek, pekk at different combinations of para and meta and it builds the molecules correctly.