Fixes #72

Fixes #82

* Change default cells.design to end of path
* Add experimental warning
* Avoid resetting cells.design

Might need a bit more work...

New exponential correction method for bcv.common estimates

Not sure if this this a good plan...

Tidy and improve examples

Handle missing (not NA) AIC values

Still some warnings from BASiCS...

Basic skeleton so far

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.