Description of proposed changes

This PR aims to add a tutorial showing how to plot focal mechanisms using pygmt.Figure.meca. It focuses on how the display of the beachballs can be adjusted and modified:

• Filling the quadrants
• Plotting the P and T axes
• Plotting the components of a seismic moment tensor
• Adjusting the outlines
• Highlighting the nodal planes
• Adding offset from the event location
• Adding a label
• Using size-coding and color-coding

Related PRs (need to be merged before this PR is mergd)

• Add aliases for L, T, Fr : PR Figure.meca: Add aliases for "L", "T", and "Fr" #2546
• Expand docstring for scale: PR Figure.meca: Expand docstring for the scale parameter #2552
• Expand docstrings for compressionfill, camp, offset: PR Figure.meca: Expand docstrings for "compressionfill", "cmap", and "offset" #2553
• Add table for conventions to docs: PR DOC / Figure.meca: Use table for supported conventions #3811
• Add aliases for Fa, Fe, Fg, Ft, Fp: PR Figure.meca: Add aliases for "Fa", "Fe", "Fg", "Ft", "Fp" #3526

Backup of code for plotting the P and T axes

# %%
# Plotting the P and T axes
# -------------------------
#
# Wait for PR #3526

fig = pygmt.Figure()
fig.basemap(region=[-size, size] * 2, projection=projection, frame=frame)

fig.meca(
    spec=mt_dict_single,
    scale="1c",
    longitude=-2,
    latitude=0,
    Fa=True,
)

fig.meca(
    spec=mt_dict_single,
    scale="1c",
    longitude=2,
    latitude=0,
    Fa="0.2c/cd",  # Compute and plot P and T axes with symbols, Adjust size and symbols
    Fe="darkorange",  # Adjust fill of T axis symbol
    Fg="gray30",  # Adjust fill of P axis symbol
    Ft="0.8p,cornsilk",  # Adjust outline of T axis symbol
    Fp="0.8p,gray60",  # Adjust outline of P axis symbol
)

fig.show()

Preview: https://pygmt-dev--2550.org.readthedocs.build/en/2550/tutorials/advanced/focal_mechanisms.html

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 wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

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