NGX export for Hippocampal Networks and TXT export for Hippocampal Networks and NeoCortex issues about neugen

[stephanmg]

1. Prefer attributes over elements.
2. Refactor public members to private and getter/setter.
3. trim whitespace from XML for disk usage minimization
4. xstream aliases for attributes
5. think about using a SAX parser within ug, or export directly .ugx from NeuGen as another export feature (see issue #13)

[stephanmg]

Implement also TXT and NGX and HOC export for HippoCampus (then cleanup!)
Cleanup TXT and NGX and HOC export for NeoCortex.

[stephanmg]

1. Write out TXT files incrementally, NGX not possible with DOM (use SAX!).
2. HOC export needs to be implemented for HippoCampal networks.

For NeoCortex fast enough!

DOM parser in NeuGen and UG limit usage of XML files (cannot use SAX parser for now!)
Use TXT export and write the TXT export incrementally! (Currently all collected in memory then written!)

This needs to be enhanced!