Allow users to modify returned frames.

[joaander]

Description

1. Internally store a copy of the initial frame.
2. Return copies of mutable entries from the initial frame and default values.
3. Ensure that all non-copied numpy arrays from the initial frame are marked non-writable.

Motivation and Context

Many users expect to modify returned frames without error:

frame = trajectory[10]
frame.particles.types = ['A', 'B']

Prior to this pull request, such changes could modify the cached _initial_frame and/or default values.

How Has This Been Tested?

A new unit test verifies that the user is given copies of the

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the GSD Contributor Agreement.
• My name is on the list of contributors (doc/credits.rst) in the pull request source branch.
• I have added a change log entry to CHANGELOG.rst.