WIP: Random field (non-)stationarity 

Proposal, following a suggestion by @gllmflndn, to store the information on stationarity of the gaussian random field as an attribute of the `search space` entity.

The main advantages are:
- The stationarity information is stored with the resel properties (`reselSize`, `searchVolumeInResels`…).
- By removing `NonStationary` from the p-value term, we prevent the storage of a `pValueNonStationaryFWER` and a `pValueStationaryFWER` in the same entity.