Updates for ORM workflows

[skearnes]

• Fix add_rdkit so it only adds molecules and fingerprints for NULL-valued cells
• Update add_datasets.py to support dataset updates (via --overwrite) and add tests
• Update the README to cover add_datasets.py

[codecov[bot]]

Codecov Report

Merging #651 (3046c06) into main (f00b6bb) will increase coverage by 0.97%. The diff coverage is 85.29%.

@@            Coverage Diff             @@
##             main     #651      +/-   ##
==========================================
+ Coverage   73.53%   74.51%   +0.97%     
==========================================
  Files          23       23              
  Lines        2713     2723      +10     
  Branches      538      540       +2     
==========================================
+ Hits         1995     2029      +34     
+ Misses        610      584      -26     
- Partials      108      110       +2     

Impacted Files	Coverage Δ
ord_schema/orm/add_datasets.py	63.63% <58.33%> (ø)
ord_schema/orm/conftest.py	95.83% <100.00%> (ø)
ord_schema/orm/database.py	90.56% <100.00%> (+1.20%)	:arrow_up:

[ipendlet]

Tests all passed on Macbook 16' 2019. Changes improved dataset workflow and management.