Change oasis writer defaults to strict + cblocks, discourage the usage of oas.gz

[stefanottili]

In order to allow multithreaded oasis reading, would be good for 0.28 to change the default options to write strict oasis with cblocks and to discourage using .gz files.

[klayoutmatthias]

Changing defaults is easy, but discouraging .gz is difficult - whenever you add ".gz" it's zipped. I don't want to skip that option entirely.

[stefanottili]

Discouraging as in pop up a warning or info message explaining that cblocks are already compressing the data.