added additional optional parameter to plot2d, now able to draw a grid and help backtracking points from the trajectories
added additional optional parameter to plot2d, now able to draw a grid and help backtracking points from the trajectories

federicomarini/flowcatchR-release,federicomarini/flowcatchR-release,federicomarini/flowcatchR,federicomarini/flowcatchR-release