Sofa FSes must be serialized before other FSes

**Describe the bug**
The serializer does not respect the following requirement from the specification:

> While the order of feature structures in the feature structures section is in general arbitrary, **it is mandatory that SofA feature structures are listed before any feature structures referring to them.** So a serializer can iterate through all the views of a CAS, then first serialize the SofA feature structure and afterwards the members of the view.

**To Reproduce**
Check e.g. the output of the tests that load XMI and serialize to JSON (e.g. `tsv3-testSimpleSlotFeature`).

**Expected behavior**
Specification should be followed.

**Please complete the following information:**
- Version: 0.5.0