Tidy and improve examples

Parameters were not being passed from the SingleCellExperiment method to the
matrix method

Still some warnings from BASiCS...

Split into functions, can now actually tell how it works...

Fix spelling

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.

Fixes #49

Fixes #64

Also remove lingering references to phenoData and featureData

Also add new BASiCS tests

Also add new BASiCS tests

Fixes #48

Fixes #48

These would sometimes error due to sampling of example dataset

Fixes #36

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