Meta issue: new data types

[ivirshup]

With #554 nearing merge-ability, I'd like to start tracking support for some new datatypes somewhere. That place is here.

datatypes to be added

• [ ] times #455
  • [ ] numpy date times
  • [ ] pandas date times
• [x] nullable pandas arrays #504

Potential additions/ possible experimental support

• [ ] Ragged arrays via awkard array #647 #609
• [ ] Ranges
• [ ] Time deltas
• [ ] xarray.DataArrays