dhis2: Implement tracker namespace file

[hunterachieng]

Summary

Implement a tracker namespaced file for tracker.import() and tracker.export() functions

Fixes #1007

Details

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[mtuchi]

Why callback support ?, i thought we are moving to a promise centric design?

[hunterachieng]

We have a branch that has removed callbacks. Until that is merged then we can fix it

[josephjclark]

hmm. To be fair this is a new API - are you sure we can't just remove the callbacks now?

[josephjclark]

I've caught some issues in the dhis2 7 branch

I was planning to do this stuff in dhis2 8, but maybe we can merge it all into the same release

@hunterachieng I do think we need to be a bit careful about doing work here while the base branch is being edited. This should be all new code so it should be well-insulated from changes - but let's just be mindful about what we're doing here, and prioritise the other branches first