Support arrays with start index != 1

[Cito]

The current implementation of the cast_array function in the C module ignores the start index of PostgreSQL arrays. They are assumed to always be 1. As discussed on the mailing list in September 2016, this should be improved:

The idea is that the cast_array function should take an additonal, optional cls parameter which will then be used as the base class for the array. If that parameter is None or list, then the method works as before, returning a list or a list of lists for multidimensional arrays. If it is any other class, then this will be considered as a subclass of list that will be used instead of the builtin list for building the return values. In addition, the instances of this list subclass will have a lower attribute set to the start index of the corresponding PostgreSQL (sub)arrays by the cast_array function.

It should then be possible to change the default value for cls passed into the cast_array function to a custom list subclass. That subclass could consider the start index when getting items, and return None it the index points to outside the array. This would emulate the behavior of PostgreSQL arrays more closely.

Note that when we support start indices when converting Pg to Py, we also need to support them when converting from Py to Pg. Currently this is done with the ARRAY constructor, which doesn't allow for start indices as far as I know, so this needs to be changed.

(copied from Trac ticket 72, created 2016-09-21)

[Cito]

Currently I see no real demand for this functionality which is difficult to implement and to use, so I have postponed it and decreased the priority.

If we're going to implement this, we should find a concrete usage example first, and use this for the tests and as an example in the documentation.

Idea: Instead of setting the lower attribute, the array parser could also pass the start index when creating the array e.g. like PgArray(lower=7).

(copied from Trac 2019-04-25)