Tidied operator implementation
Added pxd definition and testcase
Completed Row × Change multiplication and updated documentation
Trivial documentation correction
Implemented missing comparison operator tests
Added many more definitions for change C++ class
Implemented four simpler Change methods
Added more definitions for bell C++ class
Implemented three more change methods
Added bounds checks on Change methods taking bell index numbers
Implemented change::set()
Implemented missing test from upstream
Implemented multiplication of bells by changes