Move simulate_data functions to new module

The function `simulate_data` in the gaussian_process_regression module should be moved to a new module since data from the test functions can be used with other models, like neural networks.

There is an almost duplicate function in the sensitivity_analysis module as well. The two `simulate_data` functions can likely be merged.

Read the `simulate_data` implementations in the gaussian_process_regression and sensitivity_analysis modules and compare their inputs, outputs, and differences. Identify the shared module boundary and verify that both existing callers can use the merged function while preserving current behavior; run the project’s tests afterward.

Written by the indexing model from the issue text.