Added random.trapezoidal for trapezoidal distr.

Added random.trapezoidal function and associated code, modeled after
random.triangualr:
- added trapezoidal example to numpybook.lyx
- added rk_trapezoidal
- added rk_cont7, cont7_array_sc, and cont7_array to handle additional
parameters required by trapezoidal
- added test_trapezoidal unit test