Enforce collections for feature store.

[simonseyock]

This PR enforces collections for the feature store. This way the two-binding stays simple and the code can be simplified.

Fixes #653

[coveralls]

Pull Request Test Coverage Report for Build 885454541

• 22 of 27 (81.48%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.4%) to 81.056%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/data/store/Features.js	20	25	80.0%

Files with Coverage Reduction	New Missed Lines	%
src/data/store/WfsFeatures.js	2	69.06%

Totals
Change from base Build 885426217:	-0.4%
Covered Lines:	1381
Relevant Lines:	1630

---

💛 - Coveralls

[marcjansen]

This looks great to me.

If possible, start collection upgrade notes / a chnagelog here or elsewhere, so we capture the breaking changes like this one.

Please merge at will.

[chrismayer]

Code looks really good and simplifies this a lot (did not test in running the code in an application). Thanks for this!