Created CJ.dt - for cross joining two data.tables

Done as separate code on the assumption that it can later be deprecated when better code is integrated into CJ to handle two data.tables

Was initially going to be a helper function in optiRum but getting it into data.table is much more preferable.  I've not included any unit tests as the difference is significant but please see [these testthat unit tests for it](https://github.com/stephlocke/optiRum/blob/master/tests/testthat/test-CJ.dt.R) I've written for the function