Save compressed SDFGs in program folder

[romanc]

Following up on PR #2312, this PR proposes to save compressed SDFGs in the program folder. As discussed in the last meeting:

• no change to the API, i.e. we keep keyword arguments of sdfg.save() as they are.
• save program.sdfg as compressed program.sdfgz inside the program folder
• make sure that changes are backwards compatible and that the program folder is still found regardless of program.sdfg or program.sdfgz

[tbennun]

Very minor changes

[romanc]

@tbennun what do you think of the added backwards compatibility?

[tbennun]

cscs-ci run