Incorporation of PCA

[fnielsen]

Michael Riis Andersen has coded a 'PCA' artifact removal data. It is now included in the PCA branch:

https://github.com/SmartphoneBrainScanner/smartphonebrainscanner2-core/tree/pca

The code should be merged into master. There are some issues:

• Update it to use the new source reconstruction framework
• Ensure that the PCA updates correctly in blocks.
• Test?

[Styrke]

I copied Michael Riis Andersen's code into a new branch, based on master, and started discarding differences until I was left with only the essential changes that makes the artifact removal work on top of the current master branch. The result can be seen in the commits just above this comment. (8cee060 and SmartphoneBrainScanner/smartphonebrainscanner2-EEGViewer@4284973)

As noted in the commit messages i discarded a handful of nonessential changes in order to keep the diffs small and readable.

This seems to compile and run quite well for me.

[philipjhj]

At commit 4f509b4 the PCA is implemented with threshold computed from calibration data.

It seems functional, but needs thorough testing.