New exponential correction method for bcv.common estimates

Not sure if this this a good plan...

Tidy and improve examples

Basic skeleton so far

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 #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 #17