Add methods for boolean indexing of forecast member and lead time in Forecast #1159

def idx_member(self, member: int | None) -> np.ndarray:
    """Return boolean array where self.member == member"""
    if member is None:
        return np.full_like(self.member, True)
    return self.member == member

def idx_lead_time(self, lead_time: np.timedelta64 | None) -> np.ndarray:
    """Return boolean array where self.lead_time == lead_time"""
    ...