Add two new model level annotations: “maximal-causal-chain-length” and “display-as-pathway-override”

[kltm]

The two tags to add are:

“maximal-causal-chain-length” (counting number)
(other stats to follow)
automatically populated and managed by minerva as actions occur

“display-as-pathway-override” (true, false, )
does not exist without intervention? (willing to let @balhoff decide what makes most sense here)

Tagging @balhoff @vanaukenk