Cleanup parameter npz files

[bmorcos]

Parameters are transfered to the FPGA using an npz file. We should implement a system to cleanup old npz files so they don't buildup indefinitely.

[xchoo]

I put in new exit handling code to deal with the npz files as part of the refactoring / rework of the individual nengo-de1 and nengo-pynq repos (to handle the board side cleanup), and in the nengo-fpga repo to handle the host side cleanup here. If this isn't working for you, then there's an issue with how the code is working, and we should find out how to reproduce it.

[bmorcos]

Awesome. Looks like I just didn't have the latest commit locally.