Additional features

[sbihorel]

This pull request proposes:

• the addition of the tad variable type for Time after dose
• new tad-related goodness of fit plots
• new functions to set and get variables and units (including new label_units data slots to support label and unit use for simulated data)
• automated detection and use of variable label and units in scatterplots and vpc plots
• new function to apply SAS-like data formatting to convert numerical data into text-based factors