Fixing DisplayTreatment plot for data sets with large N

[adamrauh]

data sets with large N's tend to create hard to read DisplayTreatment plots due to the space between the tiles on the plot. We should add an additional graphical parameter to the DisplayTreatment function so that users can choose to remove the space between the tiles on the plot. This should be easily implemented by adding some ggplot code to generate the plot without the spaces.

[adamrauh]

fixed in this commit. Added dense.plot option to DisplayTreatment function to remove spacing between tiles to help with readability.