Minor changes for 4.1 in tub conversion script and developer doc (#708)

* Minor changes for 4.1
* Update conversion script to translate discontinuous data.
* In developer guide add disclaimer for version and correct intra-page links.

* * Update doc with donkey train command.
* Update doc with developer section for building own models in donkey 4.1
* Integrate changes from PR feedback

* Incorporate PR feedback
* Add empty records concept to tub
* Minor updates to conversion script

* Incorporate PR feedback
* Add empty record type into conversion script