Added initial nadir pointing (#332)

* Added nadir orientation calculation to frame_chain

* Added comments, replaced conditional dict access, replaced hard-coded bool for nadir parameter.

* Updated orientation to be obtained from the frame chain and added tests

* Updated the frame trace returns if nadir is true and propogated those changes to the frame chain from_spice function

* Updated focal2pixel_lines test for the time being

Co-authored-by: Adam Paquette <acpaquette@usgs.gov>