dot-getELLIPSE: Obtain ellipse coordinates from (PCO) X,Y and a factor value
In MayaGueguen/RFate: Vegetation modelling with the FATE model

Obtain ellipse coordinates from (PCO) X,Y and a factor value

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util.ellipse(mx, my, vx, cxy, vy, coeff)
util.ELLIPSE(x, y, z)
.getELLIPSE(xy, fac)

`mx`	Σ x \frac{z}{Σ z}*
`my`	Σ y \frac{z}{Σ z}*
`vx`	Σ (x - mx) (x - mx) * \frac{z}{Σ z}*
`cxy`	Σ (x - mx) (y - my) * \frac{z}{Σ z}*
`vy`	Σ (y - my) (y - my) * \frac{z}{Σ z}*
`coeff`	default `1`
`xy`	a `data.frame` or `matrix` with 2 columns corresponding to individuals coordinates, extracted from example from `dudi.pco` analysis
`fac`	a `vector` containing group labels for individuals (with `length(fac) = nrow(xy)`)
`x`	a `vector` corresponding to abscissa coordinates of individuals (column `1` of `xy`)
`y`	a `vector` corresponding to ordinate coordinates of individuals (column `2` of `xy`)
`z`	a `data.frame` with one column for each level represented in `fac` and `nrow(z) = length(fac) = nrow(xy)`. Values are corresponding to the relative representation of each level (for level `i` : \frac{1}{N_i})