FEAT: Fuzzy C-Means prediction (for independent test data)

This PR adds predictive capability to the Fuzzy C-Means clustering algorithm! Now, users can cluster data using `cmeans` and then input independent test data to `cmeans_predict`; a new fuzzy membership matrix will be generated with the membership value of each new data point for known clusters.

Also includes a fix so the random seed is only set when desired, instead of at each run (dangerous!) even if `U_init` were provided.

Further API changes and cleanup are likely.