trend_plot_hexLen: Trend plot of grid size effect
In MPE-Lab/TIPC: Tumor-Immune Partitioning and Clustering (test)

Plotting the changes in TIPC metric count as the grid size i.e. hexagon length increases; the 5 shifting directional data will be collapsed specific hex_len.

trend_plot_hexLen(
  root_dir = NULL,
  hex_len_range = NULL,
  hex_len_stepsize = NULL,
  output_dir = NULL,
  pdf_width = 18,
  pdf_height = 12
)

`root_dir`	A directory path pointing to the parent directory of the ouptput folders containing TIPC metric counts generated by `count_TIPC_cat` at various hexagonal lengths.
`hex_len_range`	A vector of 2 integer number indicating the range of hexagonal length. If unspecified (NULL), all found TIPC result folders of different hexagonal lengths will be processed.
`hex_len_stepsize`	An integer indicating the step size of hexagonal length. If unspecified (NULL), all found TIPC result folders of different hexagonal lengths will be processed.
`output_dir`	A directory path for saving output plot; if NULL, plots will be saved in root_dir
`pdf_width, pdf_height`	the width and height of the graphics region in inches. The default values are 18 and 12; inherited from `pdf`