Change to a seperate unique identifier to ID front ends within YARR

Previously the rx channel was used as a unique ID, however at least for Pixel it might be required to retrieve data for different front-ends from the same RX channel (due to data merging). Hence the unique ID which mapped all front-ends through the system had to be changed away from the rx channel.

This MR changes the unique id to an index within a vector held in the Bookkeeper which stores all front-ends. I also reworked some of the container mapping, moving most of them into the FrontEnd class itself.

As this is urgent I'm opening this MR as is, but it would be good to clean some pieces up. E.g. instead of unsigned one should probably use a different typedef for cleaner description and some of the for loops over all FrontEnds should be made neater likely using a iterator.