Add missing __doctest_skip__ directive in some methods/functions

[seisman]

Some doctests are mainly intended to demonstrate the usage of functions or methods rather than for actual testing. Such tests can be skipped using the __doctest_skip__ directive (initially introduced in PR #1790). However, it's easy to forget to add this directive to some methods or functions.

To help identify these cases, I asked ChatGPT to write a simple Python script that finds modules containing doctests but missing the __doctest_skip__ directive. It works, but still flags modules that shouldn't be reported. Thus, we can't add it to our CI workflow, but we can sometimes run it and review the report manually.

import doctest
import pkgutil
import pygmt

TARGETS = ("pygmt.src", "pygmt.datasets", "pygmt.params")

missing = []
for modinfo in pkgutil.walk_packages(pygmt.__path__, pygmt.__name__ + "."):
    name = modinfo.name
    if not name.startswith(TARGETS):
        continue

    module = __import__(name, fromlist=["*"])

    # Detect whether the module actually contains doctests
    finder = doctest.DocTestFinder()
    tests = finder.find(module)
    if not any(test.examples for test in tests):
        continue

    if "__doctest_skip__" not in module.__dict__:
        missing.append(name)

if missing:
    print(
        "__doctest_skip__ missing in modules that contain doctests:\n"
        + "\n".join(f"  - {m}" for m in missing)
    )

This PR adds the missing __doctest_skip__ directives to the appropriate methods and functions.