Fixes #69

Checks for SpatParams caused errors if parameters weren't supplied in the right
order. Fixed by making setParams a method and reordering the update list
before setting parameters.

Fixes #66

Groups (and paths) were being simulted in alphanumeric rather than number
order. Fixed by making the Group column a factor.

At request of Bioconductor core team

scater normalise renamed to normalize

Fixes #48

Fixes #48

These would sometimes error due to sampling of example dataset

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