Overlapping shells for OnionPartitioning

[JulienPeloton]

As of now, the shells use to define the OnionPartitioning are non-overlapping.
Cross-matching 2 data sets with this implementation will unavoidably lead to miss pairs at the borders.
One idea would be to make overlapping shells as

// To be in OnionPartitioning.scala, LinearOnionPartitioning(...)
// Add overlapping shells -- e.g. 1% (to be defined)
val extension : Double = 0.01
for (pos <- 0 to numPartitions - 1) {
  // Inner radius must be greater than 0 (or equal to 0)
  val innerRadius = math.max(0.0, pos * dZ * (1 - extension))
  val outerRadius = (pos + 1) * dZ * (1 + extension)
  
  val shell = new ShellEnvelope(center, innerRadius, outerRadius)
  grids += shell
}

[ChrisArnault]

Then this algo should include the rejection of the duplicates between shells

[JulienPeloton]

Not directly this algo (which just defines the grid elements), but I agree we should include in PixelCrossMatch the required method to filter cross-match duplicates.