support for data frame?

[morrowcj]

Some of the functions explicitly require matrices for some arguments, while others allow data frames. This is inconsistent and not intuitive. It would be much better if data frames could be handled. Users likely expect this.

Additionally, data.table could be useful internally for storage and operations of large data sets. These use memory references and in-place operations that are more space (and time) efficient. It may be worth considering.

---

Goals:

• support data frames (and tibbles / data.tables) as inputs
• use data.tables internally for more efficient data storage