ENH: Remove math.utils module

Remove the `math.utils` module:
- The positive semidefinite check for the kernels is built-in in
  `scikit-learn` `GaussianProcessRegressor` class:
  https://github.com/dalmia/scikit-learn/blob/598c651afd987a540e404ccdb3491d530e0b0abb/sklearn/gaussian_process/gpr.py#L245
- The `math.utils.compute_angle` function, as it was superseded by
  `model.gpr.compute_pairwise_angles`.

Author: jhlegarreta

docs/developers.rst

@@ -33,7 +33,6 @@ Information on specific functions, classes, and methods.
    api/nifreeze.data.dmri
    api/nifreeze.estimator
    api/nifreeze.exceptions
-   api/nifreeze.math
    api/nifreeze.model
    api/nifreeze.registration
    api/nifreeze.testing