Intermediate checkin, layers generalization, image slice utils
- Models:
  begin using
    p.sprout 1 for p in @patches.nOf(@population)
  rather than create agents and random x/y
- Util
  ctxTextParams->elementTextParams generalize to DOM elements
  add insertLayer for creating more layers for model
  add imageSliceToCtx for creating ctx from part of image
  factor createLayer to use insertLayer
- DataSet
  imageRowsToData added as class method; experimental
- Model
  Generalize layers to dom elements; add Image layer