Odk2stata

* When parsing choices, keep choice lists and names if the string is not empty.

* Add code for GUI

* Generated split select_multiple variable names now are one line

* Remove comma before clear in "import" statement
* Add Stata code for date in final "save"
* Fix encode select_one logic for select_external types

* Fix off-by-one error on variable numbers

* Remove commas from "foreach" lists in encode select_ones