Implement conditional region

[luca-patrignani]

This PR is part of my GSoC project. This implement ConditionalRegion, a block (not a ControlFlowRegion) which has a property branches which is a list of tuple (condition, control flow region).

Needs confirmation

• ConditionalRegion is not a ControlFlowRegion because it does not have a start block.
• The else branch is expressed as a empty region.

[phschaad]

@luca-patrignani It appears that at least one of the test cases does not terminate when automatic simplification is turned off. This is something we've encountered in the past, specifically with loop regions. The driver of the issue there was that control flow detection got confused by extraneous states (introduced through inlining) and consequently generated infinite goto-loops. Can you verify that this is again the culprit? The issue last time was in these tests: https://github.com/spcl/dace/blob/master/tests/python_frontend/loops_test.py