Model#set returns model

- to conform to Backbone.Model interface
Animation: infer default time tolerance

- Choose largest interval between times,
  if not otherwise est
whitespace, comments
Add future tile type to snow (4knam)
Update version to v0.7.19
Move SnowFallAccum to its own layer

- not really a 'future' version of snow depth
Add SnowFallAccum layer to layers package (AMD)
Merge branch 'dev' into staging
Add changelog for v0.7.19