Refactor fpga_pes_ensemble_network and DNA_extractor scripts to maximize reuse of SSH code.

[xchoo]

The fpga_pes_ensemble_network code and the DNA_extractor code contain a lot of duplicate code. They should be refactored (make an SSH class or something) to reduce code copy/pasting. Refactoring the code would also make it easier to user the same SSH communication methods for future fpga ensemble / networks.

[bmorcos]

Maybe this is also a good time to look at adding a flag to indicate that SSH has connected. Using this flag to indicate to the socket that it should start expecting values.