Avoid calling pure methods in client code

[mernst]

Chicory adds calls to pure methods to a program. Chicory assumes that calling a pure method does not affect the program's behavior. However, sometimes there is a method that ought to be pure but is not; when Chicory adds a call to such a method, Chicory changes the program's behavior.

Issue #82 shows a test that fails when an allegedly pure method is called. This can be viewed as the test assuming that no pure methods are called, or as the implementation being non-pure. Either way, this test is not compatible with Daikon's standard data observation, which adds calls to pure methods.

If this issue arises again, then we could add a new argument to Daikon that disables calling any methods in the client code. This would degrade Daikon's output, because Daikon would be given less information to generalize from. However, it would enable Daikon to be run over brittle code like the test cases in issue #82.