Fix circular import rc setting

[cvanelteren]

Closes #364

This pull request resolves a circular import error that occurred when setting the cycle parameter from a user rc file. It also includes a major architectural refactoring of the configuration system to make it more robust, extensible, and easier to maintain.

The Problem

The initial issue was caused by a circular dependency:

• ultraplot.config needed to import from ultraplot.colors to validate and process the cycle setting.
• ultraplot.colors needed to import the rc object from ultraplot.config for its own internal configuration.

This circular dependency made the application's initialization fragile and order-dependent, leading to the reported ImportError.

The solution: adding handlers to Configurator

Instead of a temporary patch, this PR implements a more fundamental solution by refactoring the configuration system to follow the Dependency Inversion Principle.

The key changes are:

1. Decoupled config and colors modules: The Configurator in config.py is now completely agnostic of color-specific logic. It no longer imports from colors.py.
2. Introduced a Handler Registry: The Configurator now has a mechanism (register_handler) that allows other modules to register their own functions for handling specific settings.
3. Moved Logic to the Responsible Module: The logic for processing the cycle setting has been moved into a _cycle_handler function within colors.py. This module is now responsible for registering its handler with the Configurator.
4. Removed Workarounds: This architectural change completely eliminates the circular dependency, allowing for the removal of all previous workarounds, including the skip_cycle parameter and the complex deferred settings logic.

Added tests

This refactoring was accompanied by a significant testing effort to ensure its correctness and prevent regressions:

1. New Handler Test: A new test file, test_handlers.py, was created to specifically unit-test the new handler registration and execution mechanism.
2. Hardened Configuration Test: The test for loading rc files (test_config.py) was completely rewritten to be more robust. After a long debugging process, the final version now directly and atomically tests the rc.load() method, ensuring the test is isolated from the user's local environment and any module-reloading side effects.

PR fixes:

• The circular dependency is resolved.
• The initialization process is now more robust, predictable, and extensible.
• The changes are thoroughly tested with both new and improved tests.

[codecov]

Codecov Report

❌ Patch coverage is 95.34884% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ultraplot/tests/test_handlers.py	86.66%	1 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[cvanelteren]

@beckermr I am pushing a more profound change that is more robust and less frail as the one pushed now. I will update the text above. One moment.

[cvanelteren]

Note that the handlers are only intended for settings that are "problematic" like the one with cycle. I think this PR is cleaner now as we separate the dependency between the configurator rand colors. The Configurator now does not need to know how colors handles the property.

[cvanelteren]

Fingers crossed that the test pass 🤞

[cvanelteren]

Will wait for the tests to pass and then merge this. Just added the API docs for this -- in case we forget in the future (likely ;-)).

[cvanelteren]

• check produced docs on docstring