Improve handling of NEST synaptic delay parameter

[clinssen]

Synaptic delay and weight in NEST are a special case and should not be treated like any other parameter (or state variable). It should be get/set only using get_delay() and set_delay() inherited from the Connection base class. A member variable is already provided by the base class.

get_weight() and set_weight() should be defined by the synapse model, but a member variable is not yet provided by the base class. We could make it such that no state or parameter marked @nest::weight is needed for minimalistic models.

When a decorator is specified in the model, the parameter can only be get/set using the name given as part of the decorator, termed the "namespace name" (i.e. "weight" instead of "w" and "delay" for "d").

As part of this PR, ASTExternalVariable was refactored out and replaced by the alternate_name (and alternate_scope) fields of a regular ASTVariable.

Fixes #707.

[pnbabu]

As these decorators are specific to NEST, could we have these in the codegen_opts for the NEST target instead?