"-1" as value for maxNumberOfPeaksPerCluster if no restriction on the number of peaks

(This pull request introduces some fixes in the FSL example used for testing at [nidm-results_fsl](https://github.com/incf-nidash/nidm-results_fsl).)

There is one point on which I would be happy to get some feedback.

Sometimes, in FSL, the number of local maxima to report (attribute `-n,--num` in [Cluster](http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/Cluster)) is not specified, which I think means that all the peaks should be reported. 

In that case, attribute `nidm:maxNumberOfPeaksPerCluster` should ideally take a value of infinity. But, as `nidm:maxNumberOfPeaksPerCluster` has range [xsd:int](http://books.xmlschemata.org/relaxng/ch19-77175.html) this is not possible (only [xsd:float](http://books.xmlschemata.org/relaxng/ch19-77095.html) can take "INF" as a value).

To circumvent this issue, I was thinking we could use "-1" as a special value that would be used to specify that there was no restriction in the number of peaks to be displayed. Do you think this is fine? Or would you prefer to model this differently?

Thank you!