Support real-valued features

[fritzo]

Support real-valued features for latent categorical and latent Gaussian states. For latent categorical states, this will correspond to a mixture of Gaussians under a normal-inverse-chi-squared prior. For latent Gaussian state, this will be a structured linear model.

• PR Implement DataTable class for heterogeneous row-oriented data.
• Implement ingest logic in format module.
• Refactor training module to use DataTable
• Refactor serving module to use DataTable
• Refactor everything else to use DataTable