v0.7.0

Added

• cgroup parameter to TrackedReference, allowing control over display and
  coloring
• group_colors in Model to allow specifying colors to use for cgroup or
  group in diagrams
• default_hide_groups in Model to specify whether to hide specific cgroup
  or group labeled references by default in diagrams
• inflow operation to allow semantically stating that a flow is actually an
  inflow to another flow (only impacts diagrams, to help indicate "flow of
  material" when a particular flow should indirectly be the inflow to a stock)

Changed

• seek_refs can now return additional "type" information for each reference in
  an equation, allowing more fine grained control during diagramming