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).

Delays 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.

For weights, get_weight() and set_weight() should be defined by the synapse model, but a member variable is not yet provided by the base class.

Because this is a NEST-specific issue, "namespace decorators" such as @nest::delay should be removed from the grammar. Instead, the weight and delay variable names should be specified as part of the code generation options.

See #707 (issue).

See #887, #996 and #995 (PRs).