Add column header setting

[liamdiprose]

This PR adds the ability to define the CSV column order from within the code
Currently, the CSV Parser assumes the first row of CSV data contains the column names.

This PR adds the ability to define the column names from the ViewSet class:

class CSVEndpoint(viewsets.ModelViewSet):
    serializer_class = CSVSerializer
    parser_classes = (CSVParser,)

    csv_header = ("time", "temperature")

There are still a few small things I would like to address before this is merged. Namely

• Is the viewset class the appropriate place to define the csv_header variable?
• Testing? I've only manually-tested this with my own project so far.
• Code style, this is quick and dirty; is there more django-y way of doing this?

Thanks