Rapid travel time plotting in taup

[johnrudge]

What does this PR do?

This pull request modifies plot_travel_times in obspy.taup to allow for rapid plotting. It does this by plotting the precalculated times in the TauModel rather than recalculating arrivals at specific epicentral distances.

The old behaviour is:

from obspy.taup import plot_travel_times
plot_travel_times(source_depth=10, phase_list=["P", "S", "PP"], npoints=50) 

The new behaviour is:

plot_travel_times(source_depth=10, phase_list=["P", "S", "PP"], npoints=None) 

where I have made npoints=None the new default, rather than npoints=50.

Why was it initiated? Any relevant Issues?

I don't think this is related to any existing issues. Recomputing all the travel times at a series of fixed epicentral distances was painfully slow, especially with a lot of phases. I've updated the relevant figures that use this function in the docs. The old behaviour is still available by setting the npoints argument to an integer value.

PR Checklist

• Correct base branch selected? master for new features, maintenance_... for bug fixes
• This PR is not directly related to an existing issue (which has no PR yet).
• If the PR is making changes to documentation, docs pages can be built automatically.
  Just add the "build_docs" tag to this PR.
  Docs will be served at docs.obspy.org/pr/{branch_name} (do not use master branch).
  Please post a link to the relevant piece of documentation.
• If all tests including network modules (e.g. clients.fdsn) should be tested for the PR,
  just add the "test_network" tag to this PR.
• All tests still pass.
• Any new features or fixed regressions are covered via new tests.
• Any new or changed features are fully documented.
• Significant changes have been added to CHANGELOG.txt .
• First time contributors have added your name to CONTRIBUTORS.txt .
• If the changes affect any plotting functions you have checked that the plots
  from all the CI builds look correct. Add the "upload_plots" tag so that plotting
  outputs are attached as artifacts.
• Add the "ready for review" tag when you are ready for the PR to be reviewed.

[megies]

🚀