Move model-specific defaults from Global.h to appropriate model class

[stiber]

Right now, there are a bunch of defaults for various parameters of LIF neurons and DS synapses. After checking that it won't break anything, these should be moved.

[stiber]

Should there even be defaults? Or should the simulator just refuse to run if parameter values aren't defined in a configuration XML file?

[stiber]

@torisalvatore, I'll take over this. I'm going to look at consistency among default values anyway.