Add rdkit extensions for reactions

[skearnes]

• Rename structure.py to rdkit_mappers.py.
• Rename rdkit.structure table to rdkit.mols.
• Rename structure attribute for Compound/ProductCompound mappers to rdkit.
• Add rdkit.reactions table and associated mappers.

[codecov[bot]]

Codecov Report

Merging #663 (230d25e) into main (556c5a8) will increase coverage by 0.57%. The diff coverage is 98.07%.

@@            Coverage Diff             @@
##             main     #663      +/-   ##
==========================================
+ Coverage   69.05%   69.62%   +0.57%     
==========================================
  Files          23       23              
  Lines        2236     2275      +39     
  Branches      565      567       +2     
==========================================
+ Hits         1544     1584      +40     
  Misses        585      585              
+ Partials      107      106       -1     

Impacted Files	Coverage Δ
ord_schema/orm/rdkit_mappers.py	92.07% <95.65%> (ø)
ord_schema/orm/database.py	92.18% <100.00%> (+1.62%)	:arrow_up:
ord_schema/orm/mappers.py	95.62% <100.00%> (+0.26%)	:arrow_up:
ord_schema/message_helpers.py	87.02% <0.00%> (+0.50%)	:arrow_up: