Performance: Speed up of deepcopy for Miller

[CSSFrancis]

Description of the change

Speeds up deep copy for Miller class. Current deep copy is slowed down by using the deepcopy function.

This causes issues in diffsims when we have to repeatedly copy and rotate some ReciprocalLatticeVector

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 orix/__init__.py and in .zenodo.json.