Replace config validation with pydantic validation

[sverhoeven]

In #324 I wanted to print a deprecation message when container_engine: singularity was used in the config file. So I wanted to add an extra validation rule.

Current validation code is copied from matplotlib and is not easy to extend. It would be nice to use pydantic.

Something like

ContainerEngineEnum(str, Enum):
  docker= 'docker'
  apptainer = 'apptainer'
  singularity = 'singularity'

class Config(BaseModel):
  container_engine:ContainerEngineEnum = ContainerEngineEnum.docker
  parameterset_dir: DirectoryPath
  parameter_sets: dict[str,ParameterSet]
  ...

  @validator('container_engine')
  def singularity_deprecated(self, value):
    if value == ContainerEngineEnum.singularity:
       warnings.warn(
            "singularity is deprecated, use apptainer instead",
            DeprecationWarning
        )

raw_config = <dict loaded from yaml string>
config = Config(**raw_config)

[Peter9192]

I would like this very much. In the example, I think the Enum object is a bit overkill, why not just use Literal['docker', 'apptainer', 'singularity']?