Adds exception handler in SparseConnection

This commit adds an exception handler to
connect_block_random function and adds some special
code to allow creation of dense (sparseness==1.0)
matrices.
Small optimization change to SparseConnection
Small reformulation in SimpleMatrix

Should improve readibilty. Have to check twice
this does not introduce a bug though.
Updates a comment in SpikingGroup
Performance optimizations in StimulusGroup

This optimizes performance for binary patterns. If binary_patterns
mode is on the Group uses a similar algorithm as PoissonGroup to
generate the spikes.
Renames extension in RateModulatedConnection

Renames connection state files from .cs to .cstate
Fixes bug in IF2Group

Fixes a problem where the NMDA voltage dependence was
implemented incorrectly.
Updates test_sync