Sampler engine structure #280

[rozyczko]

Core refactoring and shared logic:

• Introduced EngineBase (engine_base.py) as a new base class for all evaluation engines (minimizers and samplers), with centralized parameter handling, fit function wrapping, and evaluation logic.
• Refactored MinimizerBase so it inherits from EngineBase. Cleaned up the constructor and removed duplicated parameter caching logic.

Bumps integration and utilities:

• Extracted BUMPS-specific problem construction and parameter conversion functions into a new module bumps_utils/problem.py, including build_curve_problem, to_bumps_parameter, parameter_names, and parameter_snapshot. These are now imported and exposed in bumps_utils/__init__.py.
• Added a new validation module (bumps_utils/validation.py) with functions for validating run settings and input arrays

API changes and deprecations:

• mcmc_sample() now lives on Sampler class directly. The method now routes calls to Sampler(...).sample(...)
• Updated the progress_callback signature in MinimizerBase.fit to clarify that its return value is ignored and to align with the new shared engine base.