Figure out exact PES matching

[tbekolay]

The PES learning rule in the emulator and hardware both differ from the Nengo core implementation in some ways. It would be good to get a better understanding of what is different so that we can emulate it better, and choose better constants during the build process.

Specifically, we should:

• Figure out the exact Nengo core -> hardware learning rate scaling
• Figure out the exact hardware -> emulator learning rate scaling
• Vary other parameters (e.g. pre_synapse) and ensure that they work on Loihi the same as they work in Nengo core

[tbekolay]

Also a note that in order to match the PES rule in all cases, it may be necessary to generate microcode values on the fly. We currently hardcode them.

[drasmuss]

This was largely resolved in #139, I opened an issue to look at the pre_synapse behaviour specifically (#179).