Clearly define dual templates (fsaverage, fsaverageSym, and fsLR)

[tsalo]

trafficstars

Closes #1901.

Changes proposed:

• Make it clear that the so-called "dual templates" are only referred to that way by convention. For example, you could very easily combine the fsLR surface template with a volumetric template other than MNI152NLin6Asym.