Support trigger column in Forms

[jkuester]

Several years ago, ODK added support for a trigger column on xlsxforms that can be used for setting dynamic defaults as well as triggering calculations on value change. (Technically the xforms-value-changed event has been in the spec for awhile, but now has Enketo support as of 5.18.0. )

Now, technically cht-core is already using enketo-core > 5.18.0 so code changes to support this feature might not be necessary. Unfortunately, https://github.com/medic/pyxform does not include the necessary changes (from xlsx/pyxform v1.2.0 to properly support the new column.

I have logged this issue as a reminder to go back and test the trigger functionality in CHT forms once medic/pyxform has been uplifted!