Minor fixes to `NengoVertex`.
Complete `with_filters` functionality.
Modify `FilterVertex` to use new layout.
Simulator calls `pre_prepare` and `post_prepare` functions on vertices.
Migrate `TransmitVertex` to the new layout.
Minor fixes to filter functionality.
Changes to C code to account for rearranged region layout.

 - Filters move to region #14
   - 1st word is number of filters
 - Filter routing moves to region #15
   - 1st word is number of entries