def write_csv filename, opts={}
      Daru::IO.dataframe_write_csv self, filename, opts
    end
Write this DataFrame to a CSV file.

== Arguments

* filename - Path of CSV file where the DataFrame is to be saved.

== Options

* convert_comma - If set to *true*, will convert any commas in any
of the data to full stops ('.').
All the options accepted by CSV.read() can also be passed into this
function.