build_grid: Build an encompassing grid
In famuvie/breedR: Statistical Methods for Forest Genetic Resources Analysts

Build the minimal regularly-spaced grid containing a given set of points.

1	build_grid(coordinates, autofill = TRUE)

`coordinates`	two-column matrix-like set of row and column coordinates of observational units
`autofill`	logical. If TRUE (default) it will try to fill missing rows or columns with missing observations. Otherwise, will treat individuals as neighbours even if they are across an empty line.

Note that autofill = FALSE virtually removes the empty lines, considering the spacing as constant.

The parameters defining the grid, and the index of the observed coordinates in the grid seen as a vector. More specifically,

origin: the coordinates of the first (with smallest row and column values) point in the grid
step: the separation between rows and columns
length: the number of points in each dimension
idx: the index of each observation in the vectorized grid