Support for euler and derivimplicit methods added

• euler and derivimplicit methods added and validated with plasticity simulations
• spikes are written to single out.dat
• euler and derivimplicit supported on cpu as well as gpu
• mod2c added as cmake external project
• spike replay validated with bbp simulations
• multi-send spike exchange implemented

Second open source release (HBP Summit 2016)

CoreNeuron version with initial support for GPUs using OpenACC with many other new features.

First open source release

0.5.0

First open source release of CoreNeuron.