"Extras" merged into main repository

[Keaton Burns]

Hi all,

Just sending out a note that I've merged in the content of the `dedalus-extras` repository (currently my `plot_tools`, and Ben's `atmospheres` and `flow_tools`) into a submodule called `extras` in the main repository.  You can now access them as e.g.

from dedalus2.extras import flow_tools

I'd still like to maintain the distinction between application specific and plotting tools from the core code, which is why these have been placed separately into the `extras` submodule, but for now it's a lot easier to write and run examples, etc., without requiring people to have multiple up-to-date repositories in their path, etc.

-Keaton