Incorporate submodel summary statistics

[aaronwolen]

This PR adds a summary statistics table for submodel. Here's what the current implementation looks like:

Also adds several internal helper functions for working submodels:

• submodels() extracts the submodels slot from an MxModel
• smData() extracts list of observed data for each submodel
• smSummaries() calculates summary statistics for each submodel
• smN() returns the number of submodels
• smNames() returns a vectors with submodel names

[aaronwolen]

This isn't ready to merge yet—first I wanted to see if this would address #3?

[aaronwolen]

Closes #5.