Now, instead of a single module wb_trigger,
we have 3 separate modules:

1) wb_trigger_iface responsible for the low-level
interface to board and FPGA

2) wb_trigger_mux responsible for multiplexing
multiple physical triggers (external trigger signals)
to multiple, independent, trigger interfaces (FPGA signals)

3) wb_trigger responsible for wrapping 1) and 2) together
and providing a resolution method for assigning physical
triggers to trigger interfaces (e.g., fanout resolution)
and a resolution method for collapsing trigger interfaces
to physical triggers (e.g, ORing trigger interface pulses)

This fixes #56 github issue