fix pandas package feature warning (issue #19)

[psychAo]

Issue #19 reported a feature warning while using pandas package. The details can be found at (https://pandas.pydata.org/pandas-docs/stable/whatsnew/v1.3.0.html#deprecations). To sum up, users need to select only valid columns before calling the reduction function (e.g. .min, .max, .sum) on a DataFrame. So, in line 113 "df_sum = df.sum()" is replaced with "df_sum = df[["params_nt", "Mult-Adds", "Params", "Non-trainable params"]].sum()". This columns are useful to show the final table of network's information. Without redundant .sum() caculations on other columns, very very little time also can be saved : )