VECs should be optional

[insertinterestingnamehere]

Currently VECs interfere with our hardware topology layer because they have to be imported before numpy, but some other parts of the code may need to use numpy. Really all of the following should be true:

• Parla should be usable without the modified glibc and forwarding libraries if a user doesn't use VECs.
• Trying to use VECs without the supporting libraries should raise an informative error.
• VECs should be usable if the supporting libraries are loaded correctly.