Implements new indexing functions

Adds a new set of functions to SimpleMatrix to increase
readability of the propagate functions. Not clear yet if this achieves
the same performance though. So far it is only implemented in
SparseConnection.
Updates prefetching logic
Replaces types in STDPConnection

Most dynamical variables are replaced by AurynWeight variables
to allow easy switching of weight precision.
Replaces C datatypes by AurynDatatypes

This is mostly done for conformity. Some occurrances of float have
been replaced by the appropriate AurynWeight or AurynState data types.
Sets generic AurynTypes
Adds debug msgs to SparseConnection and DuplexConnection