v0.16.19

Catlab v0.16.19

Diff since v0.16.18

Merged pull requests:

• Add Sheaf implementation (#852) (@jpfairbanks)
• Floating viewer for Graphviz output (#947) (@quffaro)
• CompatHelper: bump compat for Colors to 0.13, (keep existing compat) (#948) (@algebraicjuliabot)

Closed issues:

• Error in composing StructTightACSetTransformation and StructCSetTransformation (#945)