V2: Adding example notebooks for chemprop v2.

[joelnkn]

Adding a few more notebooks to the examples directory of chemprop

[joelnkn]

Current Issues

• loaded_molecule_features.ipynb references columns of test_smiles.csv that don't exist (targets and features). This file can be replaced with any other smiles csv with both targets and features. I don't think any currently exist in the test folder.
• Both spectra prediction notebooks produce models that produce NaNs as output.