draft code for mex_model

Code that performs the hierarchical clustering on the dataset and
returns the hclust clusters when they are cut at 4.

It is likely that this code will end up being called within some master
`mex` function (see Damjan’s email).
Can use data from loaded package

Changed the code so that we are now grabbing the `sim.dat` from data()
and not a read.csv() function.
misc

Not sure why, but the description has had a space added.
exported the MCAR.test and mex_model

added the export tags.
tidy and @export

Added the @export tag and removed blank lines.
new code for mex_model

This is the new mex_model function code, at this stage it is very
simple and will be extended soon.
removed blank line

removing the blank line prevents the error: “Error : contains a blank
line”
lowercase change

Change the name of MCAR.test to mcar_test, as I think this looks nicer
and keeps the lowercase trend Hadley recommends.
minor change

ran `document()` and it removed this line.
new .Rd from `document`

Ran `document()` on the slightly changed mcar_test (previously
MCAR.test) and it reproduced the .Rd file.
change from renaming

change in the NAMESPACE from renaming the MCAR.test function to
mcar_test().
added rpart

ran the rpart function to predict the cluster from  `hclust()` when cut
point is 4.