Add a plotting helper file for producing markup files without retistruct

[briancohn]

as a user, if i do not have retistruct working properly, I should be able to take a manual route, using my data and Base-R libraries, to compose the P.csv and T.csv and M.csv so I can move forward with retinal processing and retinal plotting.

Data files required for retinal processing:

Approach: create a plot like this, and include instructions right on the plot:

[briancohn]

works on Pmol_753; need to integrate more cleanly

[briancohn]

[briancohn]

tear_markup_plot('path/to/diagram_retina/')

getting some overlap of numbers for some reason

[briancohn]

[briancohn]