Calculate an animal's volume from mass and density or length and girth

    Args
    ----
    mass_kg: float or ndarray
        Mass of animal (kg)
    dens_kgm3: float or ndarray
        Density of animal (kg/m^3)
    length: float or None
        Length of animal. Default `None` (m)
    girth: float or None
        Girth of animal. Default `None` (m)

    Returns
    -------
    vol_kgm3: float or ndarray
        Volume of animal (m^3)

def calc_seal_volume(mass_kg, dens_kgm3, length=None, girth=None):
    '''Calculate an animal's volume from mass and density or length and girth

    Args
    ----
    mass_kg: float or ndarray
        Mass of animal (kg)
    dens_kgm3: float or ndarray
        Density of animal (kg/m^3)
    length: float or None
        Length of animal. Default `None` (m)
    girth: float or None
        Girth of animal. Default `None` (m)

    Returns
    -------
    vol_kgm3: float or ndarray
        Volume of animal (m^3)
    '''
    if (length is not None) and (girth is not None):
        _, seal_vol = surf_vol(length, girth)
    else:
        seal_vol = mass_kg / dens_kgm3

    return seal_vol