Add support for inner core diffracted phases in taup with Kdiff

[johnrudge]

What does this PR do?

This pull request adds support for inner core diffracted phases in obspy.taup with the notation Kdiff. An example is:

from obspy.taup import TauPyModel
model = TauPyModel("ak135")
arrivals = model.get_ray_paths(0.0, 160, ["PKdiffP"])
print(arrivals)
arrivals.plot_rays()

2 arrivals
        PKdiffP phase arrival at 1213.904 seconds
        PKdiffP phase arrival at 1296.514 seconds

A very small tweak is also made to the plot_travel_times function updated in #3092. It better plots the wrap-around phases at 180 and 360 degrees by plotting with separate lines. Compare the far right of the new plot

with the old one of #3092.

Why was it initiated? Any relevant Issues?

Support for inner core diffracted phases has been requested for the java version of taup, see crotwell/TauP#3 . This PR supports the PKP_Cdiff phase requested in that issue, but not the PKP_Bdiff phase. @crotwell -- can you take a look and see if you want to add Kdiff to the java version too?

PR Checklist

• Correct base branch selected? master for new features, maintenance_... for bug fixes
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• Any new features or fixed regressions are covered via new tests.
• Any new or changed features are fully documented.
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[johnrudge]

Kdiff has been noted in the docs: https://docs.obspy.org/pr/inner_core_diffraction/packages/obspy.taup.html#module-obspy.taup in the Phase naming section. Over to you @megies to review.

[megies]

In principle, I think in all these cases my comment in the other ticket applies in the same way. Readability could be much improved by using named variables for the individual parts of the conditions. But I don't want to hassle you too much with it, if you feel it's OK to do it this way (I assume it might be like this throughout the taup code base..?)

[johnrudge]

I agree that the readibility could be improved, but I'm afraid I'm not inclined to do so for this PR. What I've done here done is consistent with the existing style (e.g. see how Pdiff and Sdiff are treated in lines 555-604). Really there should be some refactoring and cleaning up of the whole complicated SeismicPhase.parse_name method, and this should be done consistently e.g. whatever is done for Kdiff should also be done for Pdiff. But I think that's a big job, and requires careful thought.

I'm fine with the cosmetic change you made.

[megies]

just one cosmetics comment, just let me know if you wanna keep as is, thats fine with me too I guess