feat: plumb Dataset start/end times through graphQL (#194)

* wip

* comments and variable name change

* proposed refactor

* formatting

* revert folder move

* rename test variables

* Update AppRootQuery.graphql.ts

* implicit typecast

* fix types

* cast for now

* updte datetime type

* Update schema.graphql

* a few minor updates

* Update dataset.py

* Update dataset.py

* Update dataset.py