Let plot() accept record-by-record transparency

[seisman]

Description of proposed changes

plot() can accept a list of transparency so that each symbol can has its own transparency level.

Address #615.

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]

The feature is already implemented, and I add three new tests for it.
However, I don't want to include the three baseline images (50 KB) to the git history.

[weiji14]

Looks good. Could use a gallery example but that can be done in a separate PR.

[weiji14]

This np.c_ is new to me, but ok.