Simplify dataForPath a bit with a defer.
Add spec/view/convert modes for command line util.
Fix tag/frame definitions.

Tags and frames are now stored as allocated instances of their types
rather than values.

In particular, this fixes the lack of frames for tags, so that's another
improvement.
Provide the model's name as read from the header.
Add frame reading.
Pass channels/receivers as arguments to goroutines.

Just makes it clear that the channels are only for writing to.
Pass output channel as a write-only channel.
Simplify s32Fields decls.
Split readTriangle off into another function because it was long.

Easier to think about this way. Less nesting.
Add basic spec mode output.
Pass outputs as arguments to goroutines.