ellipses

Added ellipses because it seems to be standard in so many R functions.
My understanding of this is currently limited, except that I _think_ it
can be used to extend R functions.
imported functions

I use these functions in the mex package. I have a feeling that there
are going to be quite a few packages imported to make `mex`. I’m not
sure if that is a bad thing.
deleted line
first attempt at adding code to mex

Here I added the section that performs the hierarchical clustering then
rpart CART modelling. It runs!  I can use `mex.data.frame(sim.data)`
and it actually performs the hierarchical clustering, then the rpart
then the rpart plotting.

However, there are still problems.
1. It prints the entire output of everything that happens when I run
mex.data.frame()
2. I need to sort out my classes so that I can plot my rpart tree
3. I need to keep thinking about naming things and style.
4. I need to learn more about S3 classes.
small tweaks

used the pipe operator %>% to simplify the code so that it is easier to
understand later.

I also removed some variables that were the in rpart() call, and
replaced them with “.”, so that the rpart model runs ALL variables as
IVs (except the DV)