eliminate random crashes and erratic behaviour

[johnnyquest]

• houdini->maya translation with 'facevarying' normals/uvs sometimes work, sometimes don't (?) (is it the exporter or the importer? am I exporting these correctly?);
  • I'm not sure how the Maya importer tolerates per-point normals and uvs (meaning non-facevarying types);
  • the code uses the 'varying' type for normals/uvs that are per-point, is that the correct type?
  • I'm exporting facevarying uvs the same way as facevarying normals (ie. I don't export an indices array for them, just writing the data for each vertex and that's it). Is that correct?

Needs lots of testing to end up bullet-proof.

[johnnyquest]

(changed kVaryingScope types to kVertexScope ones as suggested by Lucas--yet to be tested then pushed)

[johnnyquest]

(Maya 2011 has dodgy behaviour when importing normals -- such as hanging for minutes or even hours. Ignore normals until 2012.)

[johnnyquest]

One type of crash is when I export from Houdini and try to import right back in (11.1.xxx). This can crash Houdini with the hdf libraries complaining about not recognizing the file, so it might be that my own compiled libs somehow clash with the ones that come with Houdini (I couldn't find any alembic link libraries in Houdini so I had to use an own compile).

If I restart Houdini then import, it's usually all right.