Add support for tidy_draws on appropriately-formatted existing data frames

[mjskay]

Maybe use an "is_tidy_draws" function. Would check for:

• .chain, .iteration, and .draw (numeric or NA)
• matrix-like format (i.e. nothing gathered into columns yet; so something like every combination of .chain / .iteration / .draw is used on exactly one row)

If the above conditions are met, tidy_draws would return the input data frame (converted to a tibble if necessary).