feat: add unit as kwarg to `Well`

[eriktoller]

I think it would be a good idea to add an optional unit="m" kwarg to the Well class.

obs = gr.Well(name="obs", is_reference=False, timeseries=obs_series, unit="m")
ref = gr.Well(name="ref", is_reference=True, timeseries=ref_series, unit="mm")

then the Well instance can convert to the SI units if needed.

class Well:
    """
    ...
    """

    def __init__(
        self,
        name,
        is_reference,
        timeseries: pd.Series | None = None,
        unit: str = "m",
    ):
        """
        Initialize a WellBase object.
        """

        # Initialize attributes
        ...

        # Check unit
        if unit != "m":
            self.update_unit()