Add df_shape

Author: savalet

include/dataframe.h

@@ -65,6 +65,7 @@ void df_describe(dataframe_t *df);
 int df_write_csv(dataframe_t *df, char const *path);
 dataframe_t *df_sort(dataframe_t *df, char const *column,
     bool (*sort_func)(void *vl1, void *vl2));
+dataframe_shape_t df_shape(dataframe_t *df);
 
 // Utils (will me moved to another header)
 char **my_str_to_word_array(const char *str, const char *separator);