Added atom underlay class
Calculation for sprout positions.
IMplemented strategh for choosing sprout positions
Atom sprouter creates atoms in the correct position.
Click to sprout from underlay
Underlay flips when bons are drawn.
Create underlay when dragging atom
Dragging from atom snaps to underlay
Don't sprout onto intersecting bonds
Underlay displays on hover and drag
Underlay is a lighter colour.
Merge branch 'underlay' into dev