Use preferred file names for nifti exported as part of NIDM-Results

As discussed in #107, this pull request implement the use of the `prov:type` value (without trailing `Map`) as preferred file name for nifti exported as part of NIDM-Results

For `StatisticMap`, the type of statistic is also included as a prefix in the file name (e.g. `TStatistic.nii.gz`, `FStatistic.nii.gz`, `ZStatistic.nii.gz`, ...), to avoid confusion when more than one statistic map is exported.