predefined viewpoints for hemibrain data

[jefferis]

@mmc46 @alexanderbates we should make a function like nview3d that works for hemibrain data (which has that extra pi/2 coordinate flip).

For example this is actually

nview3d('frontal', extramat=rotationMatrix(-pi/2, 1, 0, 0))

frontal.
@lisa-marin FYI.

[jefferis]

Could possibly teach nview3d to take a templatebrain argument that defines the current coordinate convention. Would of course mean that this needs to be recorded in the templatebrain ...

[jefferis]

Partial implementation in hemibrain_view()