Feature request: import functions

[technocrat]

It would be helpful to have functions to convert suitable csv files and ts, tsibble, tibble, data frame or data.table objects to list objects suitable as arguments to analyze and other tsmp functions.

While the feature is under consideration, a description of the attribute requirements for tsmp(...) matrices or vectors would be useful. I'm still figuring out what to make of sub_len, which is a sticking point for exploring the functionality. I'm sure there are other users also trying to find the right way of bringing their existing time series data into the package's embrace.