fix: stop using protected type as a field name on the Variable dataclass

[hollandjg]

... and update the data_type so it reads:

@dataclass
class Variable:
    """Describes an experimental variable: name, type, range, units, and value of a variable."""

    name: str = ""
    value_range: Optional[Tuple[Any, Any]] = None
    allowed_values: Optional[Sequence] = None
    units: str = ""
    value_type: ValueType = ValueType.REAL
    variable_label: str = ""
    rescale: float = 1
    is_covariate: bool = False
    data_type: Optional[type] = None