Major refactor, FTLE, more

The code has been refactored so that all key functionality is in a new "planktos" package and can be imported with a single line. All examples have been rewritten as tutorials with far more explanation and transparency as to what is going on. New examples have been added. Planktos is now capable of generating FTLE data from arbitrary deterministic motion models. Gaussian kernel density estimation has been added to plots, largely replacing histograms. Plots are now capable of displaying fluid info in the background in 2D, and the environment class has built-in plotting for sanity checking and FTLE. We are now solving SDE by default instead of ad hoc drift plus jitter; this is being done using a new Euler-Maruyama solver. A built-in Runge-Kutta Dormand-Prince solver is available for fully deterministic motion models. The doc strings of most methods have been converted to numpydoc format with far more detail and explanation about syntax, and work is ongoing to convert the rest. Finally, there are more options for VTK output and numerous bugs have been squashed throughout the code base.