STK_TESTFUN_TWOBUMPS computes the TwoBumps response function

 CALL: Z = stk_testfun_twobumps (X)

    computes the response Z of the TwoBumps function at X.

    The TwoBumps function is defined as:

       TwoBumps(x) = - (0.7x + sin(5x + 1) + 0.1 sin(10x))

    for x in [-1.0; 1.0].