creating cv_ml and m1_ml

[itchyshin]

@itchyshin to create

• [x] cv_ml - mulitlevel version of CV (heterogeneity)
• [x] m1_ml - mulitlevel version of M1 (heterogeneity)

extra stuff

• [ ] check i2_ml - it fails when the sample size is small - investigate how small

[itchyshin]

@daniel1noble - great to catch up. I pushed the first draft of M1 and CV functions - please check. Also, i2_ml no longer has the data argument.

I will check it out when boot fails a bit later

[daniel1noble]

@itchyshin Thanks! I've checked i2, updated the tests to deal with drop in data argument and updated the documentation for new functions.

Great to catch up today!

[itchyshin]

I think this is done and dusted