Avoid having to set `rate=None` explitctly when passing timestamps in `mock_ElectricalSeries`

[h-mayorquin]

Motivation

Right now you get the following error when trying to pass timestamps to the mock_ElectricalSeries function.

from pynwb.testing.mock.ecephys import mock_ElectricalSeries


mock_ElectricalSeries(timestamps=np.arange(10))
ValueError: Specifying rate and timestamps is not supported.

This is because we have rate=30,000.0 as a default value in the function signature. To make this work, the user has to explictly set rate=None, that is:

from pynwb.testing.mock.ecephys import mock_ElectricalSeries


mock_ElectricalSeries(timestamps=np.arange(10), rate=None)

This is too cumbersome. This PR simplifies this so this is not necessary. To do this, it moves the logic of default values inside the function.

I also added some missing typing.

How to test the behavior?

The code above should work.

Checklist

• [x] Did you update CHANGELOG.md with your changes?
• [x] Have you checked our Contributing document?
• [x] Have you ensured the PR clearly describes the problem and the solution?
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• [x] Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.