Add explicit user-item matrix class

[mdekstrand]

It would be good to have an explicit user-item matrix class to replace most use of CSR, that can do a few useful things:

• allow multiple components to reuse the same sparse matrix representation and user/item indexes
• handle values and presence/absence of timestamps
• enable training from different data sources (e.g. polars) instead of hard-wiring Pandas data frames as the only input data source

[mdekstrand]

This was added in #427, now we need to use it.