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.