Add functions to control E-link enables from FelixController

This MR adds functions in FelixController to enable and disable e-links by writing to the corresponding FELIX registers. An executable elinkConfig is also provided as a standalone application to check or set FELIX e-link registers based on the list of channels specified either directly via the options -t and -r, or the list of front ends in the connectivity config via -c.

Previously, we usually rely on a separate shell script that calls flx-config commands on the FELIX host to set up e-links corresponding to the ones that would be used in a scan. It should be more convenient to use the same connectivity config for elinkConfig and scanConsole for configuring the e-links necessary.

There is also a flag -e for elinkConfig that, if switched on, would also disable other e-links that are not in use. This partially addresses the issue brought up in https://its.cern.ch/jira/browse/ITKSCOMINT-52. We can potentially also add the e-link control as a part of scans, but I'm not really sure where would be the best place to put them, plus there are probably some interface changes needed.