Autoincrement fix with unit tests.  Float counter unit tests included.

This pull request includes three changes:

1. It fixes a bug where the increment parameter in the humbledb.helpers.auto_increment function is not used.  This fix makes the increment parameter operational.

2. It includes a unit test and setup code to test the increment fix.

3. It includes unit tests that expose a corner case where the sidecar document contains counters that are of type Double. The auto_increment helper function does not mask this behavior, and the unit tests are included to show this so other developers are aware that this can happen.