added Rastrigin function with unittests:

added RastriginFunction to unconstrained_functions.h
added RastriginFunctionTest to test_problems_unittest.cc

added missed ALLOW_SHARED_PTR_ONLY_CREATIONs to unconstrained_functions.h
fixed tests in test_problems_unittest.cc

Change-Id: I62bb4bcc9c6f4a82ddcfcb6da6f263d2fa590829