Handle different URCs for different modules

[sfe-SparkFro]

Copied from @PaulZC in #1:

We also need to have a discussion about how to handle this code in processURCEvent. This code is specific to the SARA_R5 and so shouldn't be in the UBLOX_AT class. We need a way to allow the SARA_R5 class to 'insert' this. Maybe we need to add a virtual method which does nothing in UBLOX_AT, but which can be overwritten by the SARA_R5 class to allow UUPSDA to be processed?

Same for this line in pruneBacklog.

[sfe-SparkFro]

This should be easy to change now that #16 has been merged