v0.18.0: Release 0.18.0

Improvements:
- various small improvements to startup time of CLI
- CLI options are grouped in help message (loam 0.8)
- option names use dashes instead of underscores (loam 0.8)
- add several melting related physical variables
- replace NamedTuples with dataclasses
- simplified installation instructions

Fixes:
- fix plots with `magma_oceans_mode` enabled but `evolving_magma_oceans`
  disabled
- matplotlib styles are only loaded when using the CLI, not in scripts
- remove unnecessary tests (e.g. redundant with mypy)

Packaging:
- drop support for Python 3.7
- add support for Python 3.11 and 3.12
- `setuptools_scm` is no longer a runtime dependency
- bump dependency versions (runtime and dev environments)

Removal:
- cached_property (Python 3.8) replaces CachedReadOnlyProperty
- `field.interpolate` option produced misleading plots, it has been removed

Internals:
- package configuration lives entirely in `pyproject.toml`
- use tox 4
- remove Makefile