Improve roc to work better with dask

[nicholasloveday]

Currently in the roc function, if you want to use dask you need to do check_args=False.

The checks need improving as:

1. Some checks are not lazy.
2. Other checks call .item() which isn't a valid method on dask arrays.

[nicholasloveday]

The same issue is occuring in the REV implementation #932