Standardised accessors, added modifiers and tools for code-gen

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This PR combines a few different enhancements that generally clean up the code and a few that are required to get my code generation code to work but which I thought would be nice to have upstream:

• Standardises child element storage/access
  • all child members are now stored in dictionaries named _aliases, _parameters, _on_events, etc,
  • dictionaries are keyed either by the name of the child element or its identifying characteristic (e.g. the trigger expression for OnCondition)
  • iterators are provided to child element values, e.g. aliases, parameters, on_events, and their keys, alias_names, parameter_names, on_event_port_names
  • lookup by key is provided for each child member type by alias('name'), parameter('name'), on_event('port_name'), etc.
• Generic lookups to child element dictionaries are provided by class_to_member dictionaries in each class that has child members, mapping the type of child element to the appropriate member dictionary. Using this I have written generic methods
  • add - add new child element to the appropriate dictionary (which is found by the type of the child element)
  • remove - removes a child element from the appropriate dictionary
  • __contains__ - tests to see whether an element (or a name) is used within the child member container
  • __iter__ - provides an iterator over all child elements (most visitors now use this iterator, which should make them easier to update)
  • index_of - generates and stores an index for a child member on demand. Useful for code generation where you would like to refer to an object via a unique index
• added methods for to each component class type that call the corresponding visitor:
  • __copy__ -> *ClonerVisitor (although not sure whether __deepcopy__ is more appropriate)
  • rename_symbol -> *RenameSymbol
  • required_for -> *RequiredDefinitions. This is a new visitor I have created to determine which expressions (aliases, ports, etc) are required for any given expression. Particularly useful for code generation

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Just checked that this branch works with the Brunel Model. To do so I had to push some more changes to my code generation and pyNN branches

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Also implemented __getitem__ for convenient access to members by name only (would not be recommended way to access them in production scripts but useful for prototyping and debugging)