Add unit tests

[felixbur]

Here is an example

• Add test parameters
• Test for expected value
• Add tests/test_to_db.py:

import numpy as np
import pytest

import spmath


@pytest.mark.parametrize(
    'x, expected_y',
    [
        (2, 6.0206),
        (.5, -6.0206),
        (0, -np.Inf),
        (-1, np.nan),
    ],
)
def test_to_db(x, expected_y):
    y = spmath.to_db(x)
    np.testing.assert_allclose(y, expected_y)