Add a Makefile target 'distclean' for deleting project metadata files

[seisman]

Description of proposed changes

See #737 for context.

Changes in this PR:

1. Do NOT delete the *.egg-info directory for make clean
2. Delete .pytest_cache (pytest cache files)
3. Add make distclean to delete all generated files and *.egg-info directory
4. Use similar syntax for the find commands

Fixes #737.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Notes

• You can write /format in the first line of a comment to lint the code automatically