This function prepares the data frame with the nationalities
to be mapped with
get_map_nats. It is used inside it.
Data frame with the statistics and the corrected nationalities.
List with the following elements:
df_all: Data frame with each country, its latitudes and longitudes and whether it must be coloured or not (depending on if there are players from that country).
countr_num: Vector with the countries from where there are players and the number of them.
leng: Number of countries in the world.