Set a default gzip compression level globally

[rhpvorderman]

This simplifies the previous setup where the default was determined by the backend.

I set the default to 1 for now. Most people will have python-isal installed, and this would have been the default for None-level compression for most users. I prefer promoting python-isal anywhere possible and as xopen states on the README that it chooses the most efficient way possible a level of 1 seems appropriate.

I am also open to setting it to 5, however. I don't think it is that important. As most upstream applications have their own preferred default anyway.

[rhpvorderman]

I forgot to mention, but this started as a refactor because I think the _open_gz function got a little messy. Setting a global default for compression level allowed removing a lot of code.

[marcelm]

Looks good! (Only one complaint about language ...)

I haven’t checked in detail whether the logic is still equivalent, but I’ll trust you and the tests.

[rhpvorderman]

I haven’t checked in detail whether the logic is still equivalent, but I’ll trust you and the tests.

I set a default threads at one point, but had to conclude that this changed the logic and changed it back. Also the reason to choose level 1 by default is that currently python-isal is chosen for compression when it is available. Setting it to 5 or 6 would change this behavior. So I think the logic is still equivalent, except that level 1 is chosen rather than level 2 (python-isal default) but that is OK (and more consistent).

[marcelm]

Thanks!