Introduce plot_regen_proportion() function for regeneration proportion and DBH checks

[pkalanta]

This pull request introduces the plot_regen_proportion() function.
The function:

• Computes regeneration proportions by MeasureID, plot, and source
• Identifies partial and high regeneration measurements
• Produces a histogram of regeneration proportions by source
• Computes elapsed time between measurements
• Evaluates DBH growth against expected values and flags implausible growth

[ianmseddy]

Hi Parvin - can you remove the histogram and ggplot changes, and then regenerate the documentation for the package?
In addition, rather than use that sapply line to generate the previous year, you can do this

plotShift <- unique(DT[, .(OrigPlotID1, MeasureID, MeasureYear)])
  setkey(plotShift, OrigPlotID1, MeasureYear)
  plotShift[, previousMsrYear := shift(MeasureYear, n = 1), .(OrigPlotID1)]

It will return the previous MeasureYear, NA if there is none.