plot_pca needs to support more than 6 replicates

[hsiaoyi0504]

Currently, it only has 6 shapes available in the static (ggplot2) plot.

FragPipeAnalystR/R/pca.R

Line 304 in 53a9fa4

shape = pca_df[[indicate[2]]]

[hsiaoyi0504]

See similar solution on StackOverflow: https://stackoverflow.com/questions/26223857/more-than-six-shapes-in-ggplot.