Improvements to Conditional Join (#943)

* updates

* update

* multiple conditions

* testing an idea

* for loop goes only one step up and after that a blow up;  opportunity for future optimizations

* split multiple conditions into smaller functions; added more tests

* Add notebook example

* Edits to test_conditional_join

* Edits to docs

* edit docs - no support for string columns

* edit docs - no support for string columns

* fix failing tests

* Allow joins on string and category

* Add more tests

Author: samukweku