a simple wrapper to compute K factors and add rhoa

def _add_rhoa(df, spacing):
    """a simple wrapper to compute K factors and add rhoa
    """
    df['k'] = redaK.compute_K_analytical(df, spacing=spacing)
    df['rho_a'] = df['r'] * df['k']
    if 'Zt' in df.columns:
        df['rho_a_complex'] = df['Zt'] * df['k']
    return df