WIP: Terms update according to spreadsheet

Companion pull request for the recent changes applied to the [Neuroimaging terms spreadsheet](https://docs.google.com/spreadsheets/d/16pC2cDsdxlzv2CzlNMtStqt5-xHwDEsU6MjZVxWhrE4/edit?usp=sharing).

Review of the changes:
- Generic terms moved from `spm` to `nidm` namespace: `ContrastEstimation`, `Inference`, `
ModelFitting`, `userSpecifiedThresholdType`, `excursionSet`, `pValueFWER`, `qValueFDR`, `clusterLevelStatistic` and `peakLevelStatistic`.
- Specific term moved from `nidm` to `spm` namespace: `softwareRevision`
- Deleted attribute: `spm:standardErrorMap` as there is no standard error map natively in SPM.
- Renamed:  `clusterLabelsMap` (instead of `clusterLabels`), `nidm:clusterLabelId` (instead of `nidm:labelId`), `maskMap` (instead of `subVolumeMask`)
- `statisticType` replaced by a `prov:type` attribute as suggested by @nicholsn on NIDASH-DDWG call 07/04/2014. Similarly for `statisticalTest`.