Add common alias transparency (-t) to all plotting functions

[seisman]

Description of proposed changes

Add transparency (alias of -t option) to all plotting modules.

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]

transparency can also be an array for a few modules, e.g., plot, plot3d and text, so that symbols and texts can have varying transparency. It needs more work and should be addressed in separated PRs.

I will open a feature request after merging this PR.

[weiji14]

Ok, but could you add at least one unit test for transparency on a module that won't have array-like transparency inputs (e.g. image)?

[seisman]

It means adding another baseline image to the repository. 😄

[weiji14]

Maybe just a gallery example then, if not too much work 😶 I suppose we could come up with a smart check_figures_equal test but happy to leave that to the next PR.

[seisman]

There is an upcoming example in #607, but I'm not sure if the example does too many things (symbols, transparency and legends.).

[weiji14]

Ah right, missed the transparency part on that example. I don't think we can have too many examples, if anything we have too little right now (a lot of catching up to do with matplotlib at https://matplotlib.org/gallery/index.html)!

[weiji14]

Cool, I haven't actually used transparency before, but looks fine. An example would be nice 😉

[weiji14]

Ok, but could you add at least one unit test for transparency on a module that won't have array-like transparency inputs (e.g. image)?