Add tabular preprocessing

[ahzero7d1]

Initial implementation of tabular data preprocessing

Closes #1062, fixes #1063

Changes

• Add validation on the web app side to reject datasets containing: nan values, empty strings
• Implement standardization for numerical features
• Store standardization metadata as part of model serialization

TODO

• Add one-hot-encoding for non-numerical features

[JulienVig]

Missing things required before merging (for me to handle):

• Use metadata statistics during model validation
• Current titanic sample csv is rejected by webapp because of missing data
• Standardization is done locally. This can lead to different weight scales between clients. Should the standardization statistics be averaged by the server?
• Unit tests for new tabular preprocessing functions
• Validate CSV right away when connecting it
• update docs/examples/custom_task.ts
• throw error if categorical features have missing data instead of creating a new category