This PR modernizes the Hydro-Without-Hydro Start-to-Finish Notebook

[leowerneck]

This PR contains the following updates:

• diagnostics_generic/output_yz_or_xy_plane.py: extended plane diagnostics function to accept a file pointer;
• Tutorial-Start_to_Finish-BSSNCurvilinear-TOV_Initial_Data.ipynb: modified to reflect the first change;
• Tutorial-Start_to_Finish-BSSNCurvilinear-Exact_Initial_Data.ipynb: modified to reflect the first change;
• Tutorial-Start_to_Finish-BSSNCurvilinear-Neutron_Star-Hydro_without_Hydro.ipynb: minor modernizations to the main function; notebook now works in coordinates other than Spherical-like.