Make tidy3d components immutable

[tylerflex]

To do list:

• Make front end immutable, passing all tests.
• make Tidy3dBaseModel.__config__.frozen = True
• copy with update.
• handle caching of properties automatically assuming immutability (simplified).
• test construct() to see if it works.
• fix any backend tests that fail.
• update all notebooks.
• update docs with FAQ section about this, why we did it, what changes.