Be a bit more careful when linking to index variables and allow some references to pre/post variables to not be available in synapses because of too complicated indexing
Fix an issue surrounding scalar variables and referenced subexpressions. Also explicitly raise a NotImplementedError when trying to use a linked variable with a subexpression -- before it failed with an opaque error message
Merge pull request #277 from brian-team/linked_var_synapses_fix

Be a bit more careful when linking to index variable