Update value for originalFileName attributes

This pull request follows discussions at #108, #114 (datatype) regarding the value taken by the `originalFileName` attributes.

Summary of the changes:
 - The `originalFileName` attribute is meant to keep a link with the full provenance and therefore stores the name of the file before export to NIDM (e.g. for `ContrastMap` in spm: `con_0001`, in fsl: `cope1`)
 - The filetype is omitted in the `originalFileName` to avoid confusion with the filetype of the file that is stored in the `prov:location` attribute (and will be stored elsewhere and dealt with in another pull request).
 - The `originalFileName` attribute is omitted if the file was especially created for the NIDM export, i.e. in  `DesignMatrix`, `ExcursionSet` (SPM), `SearchSpaceMap`, `SubVolumeMap`, `CustomMaskMap`, `GrandMean` (SPM), `ContrastStandardErrorMap`, `ResidualMeanSquaresMap` (SPM).

Could you let me know if this looks like what you expected following our discussions? I am happy to update as needed.