Enable buffer plots in designs

[rogerssam]

Enable different options for buffer plots in the design() function. Options could be "edge", "plots", "rows", "columns", "double row", "double column", "blocks", maybe more?

[rogerssam]

Cases:

• edge: one extra row top and bottom, and column left and right. nrow + 2, ncol + 2
• rows: extra rows in between and top and bottom. (2 *nrows) + 1
• columns: extra columns in between, plus left and right. (2 * ncols) + 1
• plots: every plot is surrounded by buffer plots in every direction. Basically combination of rows and columns above. (2 *nrows) + 1, (2 * ncols) + 1
• double row: every treatment row has an additional buffer row above and below, so treatment rows separated by 2 buffer rows between. (3 * nrow)
• double column: every treatment column has an additional buffer column left and right, so treatment columns are separated by 2 buffer columns between. (3 * ncol)

[rogerssam]

This should be done as part of the design function, not the plotting function.