Add with_polymorphic mappers for query construction

[skearnes]

• Add with_polymorphic mappers like Compound to refer to any child of _Compound.
• Add RDKit cartridge functionality into a separate rdkit schema in the database.
• Helper functions for initializing and populating the database.

(Make sure you hide whitespace when viewing the diff, or it will look much larger than it is!)

[codecov[bot]]

Codecov Report

Merging #645 (23d2b34) into main (553c67e) will increase coverage by 0.91%. The diff coverage is 97.95%.

@@            Coverage Diff             @@
##             main     #645      +/-   ##
==========================================
+ Coverage   73.11%   74.02%   +0.91%     
==========================================
  Files          17       20       +3     
  Lines        2462     2591     +129     
  Branches      655      666      +11     
==========================================
+ Hits         1800     1918     +118     
- Misses        557      567      +10     
- Partials      105      106       +1     

Impacted Files	Coverage Δ
ord_schema/orm/database.py	87.17% <87.17%> (ø)
ord_schema/orm/structure.py	93.33% <93.33%> (ø)
ord_schema/orm/__init__.py	95.00% <95.00%> (ø)
ord_schema/orm/mappers.py	99.17% <99.17%> (-0.16%)	:arrow_down:
ord_schema/message_helpers.py	85.63% <100.00%> (ø)