Create function to save population

[coopa33]

• We would like to have a function which is able to save populations at certain checkpoint. The function should save the population as some sort of file, locally on the computer.
• Ideally, the names of the files saved should be unique, so that they don't overwrite each other (see save_unique_png() in A3_modified.py for such a logic)
• If we have the file, we then also need a decoder that can decode the file into a list of genotypes again.