Improve json serialization to accomodate numpy float32

[YuanTingHsieh]

Since numpy 2.0, some type promoting rules are different: https://numpy.org/devdocs/numpy_2_0_migration_guide.html#changes-to-numpy-data-type-promotion

Description

• Change to always cast np.float32 to np.float64 (for serialization) before calling json.dump

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Quick tests passed locally by running ./runtest.sh.
• In-line docstrings updated.
• Documentation updated.

[YuanTingHsieh]

/build