Arrangements: Unify mix of pre and post condition macros

[afabri]

Summary of Changes

We had postcondition_code() followed by precondition() . I saw it when I only switched off postconditions with a macro. We might do that for an entire testsuite.

Release Management

• Affected package(s): Arrangement on Surface

• License and copyright ownership: unchanged

[sloriot]

Successfully tested in CGAL-6.0.1-Ic-351