Post-release v0.6.1 updates

[hakonanes]

Description of the change

Add Unreleased section to changelog and set version to 0.7.dev1.

Progress of the PR

• [n/a] Docstrings for all functions
• [n/a] Unit tests with pytest for all lines
• [n/a] Clean code style by running black

For reviewers

• The PR title is short, concise, and will make sense 1 year later.
• New functions are imported in corresponding __init__.py.
• New features, API changes, and deprecations are mentioned in the
  unreleased section in CHANGELOG.rst.
• Contributor(s) are listed correctly in credits in diffsims/release_info.py and
  in .zenodo.json.

[hakonanes]

Fairly uncontroversial changes here, merging myself.