Add load_kernels method to TourConfig, Trajectory and MetaKernel objects

Implement load_kernels methods into TourConfig, Trajectory and MetaKernel objects:

>>> tour = TourConfig(mk='metakernel.tm', load_kernels=True)
# or
>>> tour.load_kernels()

>>> traj = tour[...]
>>> traj.load_kernels()

>>> mk = MetaKernel('metakernel.tm', load_kernels=True)
# or
>>> mk.load_kernels()

See docs.

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Closes #62 (closed)