Update symmetry tutorial with simulations

[henneysq]

This pull request adds details to the tutorial on beamformer source estimation with symmetry constraints to show the effects of this on correlated sources.

[robertoostenveld]

Instructions for running the website locally are documented here https://github.com/fieldtrip/website/blob/master/README.md#evaluating-the-website-locally

I hope you can get that to work. I use rvm to bootstrap the Jekyll stack.

[henneysq]

@robertoostenveld do you want me to add some details to the real-data example based on Popov2018?

[henneysq]

STATUS:
The symmetry simulations tutorial now renders in its entirety and contains four examples. I can merge all the code into two for loops instead to make it less comprehensive, but I think this sequential structure is easier to follow as a tutorial.

I will add some descriptions to the plots, and feel free to do the same if you want to attach any comments.

[robertoostenveld]

Hi Mark, pointing to Popov2018 would be useful. If you feel it looks more or less OK, I suggest to merge, then check out how it looks online (which I will do as well) and then make further small changes.

[henneysq]

Hi Mark, pointing to Popov2018 would be useful. If you feel it looks more or less OK, I suggest to merge, then check out how it looks online (which I will do as well) and then make further small changes.

Okay. I will break down and describe the ASSR_lcmv.m script from Popov2018, then mark the PR ready to merge.

[henneysq]

I took the ASSR_dics.m from Popov2018 apart and described the content for a single subject.

[robertoostenveld]

let me merge, and let's then have a look at it online.