Adding information in the readme and in the tests

import math

import numpy as np

import xarray as xr

from pygridsio import plot_grid

import matplotlib.pyplot as plt

        results_p_value = results.isel(p_value=i_p_value)

        for i_variable, variable in enumerate(results.variables):

            if variable in ["p_value", "x", "y"]: continue

            plot_grid(results_p_value[variable], axes=axes[i_variable - 3, i_p_value])

            if axes.ndim == 2:

                ax = axes[i_variable - 3, i_p_value]

            else:

                ax = axes[i_variable - 3]

            plot_grid(results_p_value[variable], axes=ax)

    plt.suptitle(title)

    if outfile is not None: