WIP: Parent classes for value of ObjectProperty

Please note that this pull request is based on #119 (PR #119 should be reviewed first).

---
Following discussions at #96, this pull request creates new parent classes for the classes used as values for Object Properties:

As agreed in #96 for worldCoordinateSystem:
- `WorldCoordinateSystem` class is created (parent of `MNICoordinateSystem` and `IcbmMni152NonLinear6thGenerationSpace`).
- `woorldCoordinateSystem` attribute is renamed in `inWorldCoordinateSystem` to avoid confusion with the class name.

Following discussion at #81 for alternativeHypothesis:
- [STATO_0000208: "alternative hypothesis"](http://bioportal.bioontology.org/ontologies/STATO/?p=classes&conceptid=http%3A%2F%2Fpurl.obolibrary.org%2Fobo%2FSTATO_0000208), [STATO_0000286 "one-tailed test"](http://bioportal.bioontology.org/ontologies/STATO/?p=classes&conceptid=http%3A%2F%2Fpurl.obolibrary.org%2Fobo%2FSTATO_0000286) and  [STATO_0000287: "two-tailed test"](http://bioportal.bioontology.org/ontologies/STATO/?p=classes&conceptid=http%3A%2F%2Fpurl.obolibrary.org%2Fobo%2FSTATO_0000287) are re-used from STATO.
- Similarly to what was done for `inWorldCoordinateSystem`, `alternativeHypothesis` attribute is renamed in `hasAlternativeHypothesis`.
 
Following discussions at #96 for statisticType:
- [STATO_0000039: "statistic"](http://purl.obolibrary.org/obo/STATO_0000039), [STATO_0000282 "F-statistic"](http://purl.obolibrary.org/obo/STATO_0000282) and  [STATO_0000176: "T-statistic"](http://purl.obolibrary.org/obo/STATO_0000176) are re-used from STATO.
- `nidm:ZStatistic` is created as a child of `STATO_0000039 "statistic"`.

This is work-in-progress and it would be great to have some comments about your views on how to proceed with those changes. In particular:

1. Regarding the re-use of terms from STATO. Shall we create synonyms in the nidm namespace and use `rdfs:sameAs` to point to the STATO definitions or would it be preferable to re-use directly STATO's terms? I am not sure about the implications of these two options. For PROV we reused directly the PROV terms but as STATO's uses numerical identifiers we might want to duplicate them in nidm? @nicholsn, @khelm, @JessicaTurner: would you like to comment on this?

2. `ZStatistic` is currently in the fsl namespace while `TStatistic` and `FStatistic` are in nidm namespace. While a ZStatistic map is only found in fsl, I was wondering if we could move this term to the nidm namespace (as a ZStatistic is a generic concept)? @nicholst: would you agree with this?

All comments are welcome, thank you in advance!