python data types revisited!

[azamifard]

@aperrin66
@akorosov

As mentioned in nansencenter/django-geo-spaas-harvesting#40 Python lists are only needed when we want to save the order of elements inside it. When we don't need order, there is no need to use a python list!!!!

Sets should be used for the case that we add another element in it afterwards.
tuple should be used for the case that we do not need to change it afterwards.

For the speed of running the code the order of selection data types is as follows:

1.tuple
2.set
3.list