Create sDCM tutorial for fMRI

[david-hofmann]

Create a tutorial for spectral DCM based parameter inference of fMRI signals:

• simulate a three region system with a OU process as input
• estimate the CSD by fitting a vector autoregressive model
• estimate the parameters
• produce plots that resemble those in papers and SPM12

[david-hofmann]

The balloon model is not a stable model. The underlying linear system is simulated in a stable regime but the balloon model produces runaway dynamics. So far I was able to stabilize it by setting the coupling between neural activity and balloon model to 0.01.
Can we figure out a better solution? Is stability of the model documented somewhere?

[david-hofmann]

Meanwhile the necessary plotting recipes are implemented in Neuroblox/Neuroblox.jl#412. The above issue is still open but plan to circumvent by using the exact same SPM12 parameters for now.

[david-hofmann]

Started PR #21 . Need to create a recipe for cross-spectral densities.