Fix path handling in notebook runner to allow use with IPTA DR3

[rossjjennings]

This makes the path handling in the notebook runner a bit more flexible, in a way that should make it possible to do notebook autoruns for IPTA DR3 and other future datasets where we use pint_pal as a standalone package. None of the changes should affect the way we had been doing notebook autoruns for the NANOGrav 15-year dataset.

Specifically, the changes are:

• run_in_subdir(), which contained substitutions specific to the 15-year process notebook as well as functionality to create a new subdirectory and run the notebook in it, has been eliminated. The 15-year-specific substitutions have been moved to test_run_notebook(), and the subdirectory functionality has been moved to run_notebook(), which I renamed run_template_notebook().
• run_template_notebook() no longer overrides the tim-directory and par-directory specified in config files. Instead, if they are not absolute paths, it treats them as relative to the parent directory of the directory containing the config file, which is a convention we have continued to adhere to for IPTA DR3.
• run_template_notebook() can now accept relative paths for the template_nb and config_file arguments, in addition to absolute paths. For template_nb, the path is treated as relative to the pint_pal/nb_templates directory, while for config_file, it is treated as relative to the current working directory.

[rossjjennings]

The test failure doesn't seem to be related to this PR specifically. It looks like it's failing to download some packages related to Ghostscript.

[rossjjennings]

OK, I think I fixed the previous failure, which was related to a missing "apt-get update" in GitHub Actions. The new failure is legitimately a problem with my changes.

[rossjjennings]

It turned out I had to change the par-directory and tim-directory specified in PINT Pal's test config files. These were "wrong" before this change, too, but previously it didn't matter because they were overridden by run_notebook() anyway.

[swiggumj]

Thanks for taking care of this Ross, and for having a look, Joe.