Improve Makefile to clean __pycache__ directory recursively

[seisman]

Description of proposed changes

Running make clean only deletes the pycache directory in the
project root. There are still a few pycache directory left in the
subdirectories.

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.

[seisman]

@weiji14 Just a quick off-topic question: should we use milestone 0.2.1 instead of 0.2.x? I think 0.2.1 is easier for us and others to track the changes in a single release. If we decide to release 0.3.0 instead of 0.2.1, it's also much easier to changes all 0.2.1-related PRs/issues to 0.3.0.

[weiji14]

Looks good, didn't realize there were so many nested pycache directories!

One thing that bugs me is that the Makefile commands are very UNIX specific (i.e. it works on Linux/macOS, but not on Windows). But that's for a separate PR.

@weiji14 Just a quick off-topic question: should we use milestone 0.2.1 instead of 0.2.x? I think 0.2.1 is easier for us and others to track the changes in a single release. If we decide to release 0.3.0 instead of 0.2.1, it's also much easier to changes all 0.2.1-related PRs/issues to 0.3.0.

No strong preference, happy to go with it (though I do like that milestone v0.2.x is at https://github.com/GenericMappingTools/pygmt/milestone/2 and milestone v0.3.x is at https://github.com/GenericMappingTools/pygmt/milestone/3, but that is bound to break at some point).

[seisman]

One thing that bugs me is that the Makefile commands are very UNIX specific (i.e. it works on Linux/macOS, but not on Windows). But that's for a separate PR.

I believe we will fix it when a Windows developer joins the team.

No strong preference, happy to go with it (though I do like that milestone v0.2.x is at https://github.com/GenericMappingTools/pygmt/milestone/2 and milestone v0.3.x is at https://github.com/GenericMappingTools/pygmt/milestone/3, but that is bound to break at some point).

OK. I'll change the milestones later.