BaseExperiment abstract methods not enforced without ABC

[drbenvincent]

Description

BaseExperiment uses @abstractmethod decorators but does not inherit from ABC,
so Python does not enforce abstract methods. This allows instantiating incomplete
experiment subclasses without errors, which can lead to runtime failures later.

Steps to Reproduce

1. Define a subclass of BaseExperiment without implementing abstract methods.
2. Instantiate the subclass.

Expected Behavior

Instantiation should fail with a TypeError until all abstract methods are implemented.

Actual Behavior

Instantiation succeeds because BaseExperiment does not inherit from ABC.

Environment

• CausalPy version: 0.7.0 (or current main)
• Python version: 3.11+
• OS: macOS (darwin)

Proposed Solution (if known)

Make BaseExperiment inherit from abc.ABC to enforce abstract methods.

Additional Context

Relevant file: causalpy/experiments/base.py.