Prefixlen lookups should be handled all in one, rather than three separate lookups

[tarkatronic]

As brought up in #61:

This brought to mind a thought: Instead of having 3 lookups: __prefixlen, __max_prefixlen, and __min_prefixlen, wouldn't it make sense to be able to just have a single __prefixlen and be able to chain other operations onto it? So for example you could have:

• MyModel.objects.filter(network__prefixlen=24)
• MyModel.objects.filter(network__prefixlen__gte=24)
• MyModel.objects.filter(network__prefixlen__lte=24)

I don't recall the exact implementation details for something like this, but what are your thoughts?

Currently, using __gte or __lte produces the following error:

django.core.exceptions.FieldError: Unsupported lookup 'prefixlen' for CidrAddressField or join on the field not permitted.

[adamcik]

I've been out of the loop for Django ORM things for ages, but turning prefixlen into a https://docs.djangoproject.com/en/1.10/ref/models/lookups/#transform-reference looks promising.

[adamcik]

django-postgresql-netfields/netfields/lookups.py

Line 125 in 9837403

class Family(Transform):

already does this for family, so I think just replacing the Prefixlen lookup with the same setup as for Family would do the trick. Then just keep the max_prefixlen / min_prefixlen around for backwards compatibility or something :-)

[tarkatronic]

Huh, strange. family seems an odd one to me to be a transform. But yes, that does look like the trick. Thanks, @adamcik! I'll get on making that change, and probably even put in a DeprecationWarning for the max_prefixlen and min_prefixlen. This will add some "fun" tests!