Splicing Docs

[robelgeda]

Related to #254
This PR adds a Splicing section to the docs.

[eteq]

@robelgeda - while reviewing this I realized I wanted to see the plot of the result of this example, so I tested it out locally (see the commit I pushed up for how I implemented adding a plot to the end of what you wrote). In the process, though, I realized what was implemented was not actually a splice - splicing a spectrum means the two pieces of the spectrum should be non-overlapping, which was not the case in your original example. It was a small modification to what you had to adjust the grids a bit to make it a splice though. So I just did this in 2f9ac1b and pushed it up since my understanding is that you are not free to work on this for the next few weeks. But I think with my modifications this is now ready assuming the tests pass.

[robelgeda]

Thanks a lot @eteq!

[eteq]

The failures are unrelated and due to my re-running the tests, but the important ones all passed, so merging!