Allow reading CSV as an iterator

[JD557]

The CSV read method (https://github.com/twosigma/beakerx/blob/dbe869b27e641095db6ebc3dee4b8e66104adb1c/kernel/base/src/main/java/com/twosigma/beakerx/fileloader/CSV.java) only allows reading CSVs to a List.

This makes parsing large CSVs impossible, even if the code is something like new CSV().readFile("./large-file.csv").take(10).toList.

Adding a readIterator method would fix this (and it would also be possible to add safer methods in Scala, such as streamFile: Stream[Map[String, AnyRef]]).

[LeeTZ]

Thank you for the contribution! @JD557