calc_SpatialIndex: Calculate spatial indices for points in 2D space
In niknap/MeanShiftR: Tree delineation from lidar using mean shift clustering

Function that returns spatial indices (plot numbers) for given coordinate pairs. E.g. for hectares res = 100 m

calc_SpatialIndex(
  xcor,
  ycor,
  res = 1,
  minx = NA,
  miny = NA,
  maxx = NA,
  maxy = NA,
  exclude.maxx = T
)

`xcor`	X-coordinate of the point
`ycor`	Y-coordinate of the point
`res`	side length of one spatial subunit
`minx`	minimum X-coordinate
`miny`	minimum Y-coordinate
`maxx`	maximum X-coordinate
`maxy`	maximum Y-coordinate
`exclude.maxx`	boolean to set whether the given maxx should be excluded or included in the indexing