The core clustering code has been completely re-written and is now far more efficient.

- Clustering is typically about ten times faster now.
- Due to changes in how chi-squared calculations are now made and
combined clusters are tracked along the way, potential row combinations
that are “tied” (or have chi-squared reductions that are so extremely
close as to be essentially tied) may be clustered in a different order
compared to the version 1.0 function.
- This generally only affects earlier clustering steps, well above where most cut points will be in practice.
- If results from a previous version 1.0 clustering need to be
reproduced exactly for every step, you can force the use of the old
function by calling
`greenclust:::.greenclust.v1()`

. This feature is not guaranteed to remain in future versions.

Verbose output now includes the names of the two rows that were combined at each step.

Fix for passing data frames to

`greenclust()`

(#8)