Default CNSGA Offspring Filename Not Valid on Windows

[electronsandstuff]

Hey folks. I was using data from the CNSGA generator recently and discovered that the default name for "offspring files" contains the colon character which is reserved on windows filesystems (https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file).

Here is the section of code which writes the file.

    def write_offspring(self, filename=None):
        """
        Write the current offspring to a CSV file.

        Similar to write_population
        """
        if self._offspring is None:
            logger.warning("No offspring to write")
            return

        if filename is None:
            filename = f"{self.name}_offspring_{xopt.utils.isotime(include_microseconds=True)}.csv"
            filename = os.path.join(self.output_path, filename)

        self._offspring.to_csv(filename, index_label="xopt_index")

Here are some examples of files that are getting generated.

cnsga_offspring_2024-08-30T14:04:59.094665-05:00.csv
cnsga_offspring_2024-08-30T14:09:29.129272-05:00.csv
cnsga_offspring_2024-08-30T14:13:17.169997-05:00.csv

[roussel-ryan]

Addressed by #251