Add T1, IBNWB, etc. brain surfaces

[jdmanton]

[jefferis]

Have sent @ajdm a T1surf-ascii.surf which should be suitable for T1, which is likely to be the main one that people will want (since it is the template for the VT dataset).

[jdmanton]

T1 surface added in bc5d69a.

[jdmanton]

I'll sort out IBN and IBNWB on gjac, when it's free. Are there any others we should add?

[jdmanton]

I'm having trouble with IBN/IBNWB in that the isosurface that Amira produces is not great, even with downsampling. It looks like the fairly variable 'staining' of the brain means that it's hard to produce a good isosurface, especially in that if you want to have decent optic lobes you need a low threshold, to avoid holes, but this then means there's lots of rubbish included near the central brain.