Update calculate_MSSI.R

Correction for R warnings :
%.% is deprecated.  use %>% instead
Update calculate_MSSI.R

Net displacement in dataframe nd is column 8 and not 7 (or I'm mistaken)
Update calculate_MSSI.R

There is no disp column in gd but a diff_y column
Update calculate_MSSI.R

Problem with column in disp dataframe. Column 7 is diff_y and we want column 8 which is disp.
Corrected line 85 where it was indeed disp and not diff_y
Merge pull request #1 from rpatin/patch-1

Update calculate_MSSI.R