Fully implement the timestep limiter (non MPI)

This is a large new chunk of code. This implements the following changes:

• Implement the time-step limiter condition based on differences in time-bin and not on differences in the signal velocity,
• Move the time-step limiter loop after the time-step task,
• Add a synchronization runtime flag (--sync) that switches on the synchronization of particles hit by feedback to the current time,
• Move the time-step limiter infrastructure out of the individual hydro schemes and into its own set of files,
• Make the time-step limiter loop a loop of type "1" and not type "2" as required by the original scheme.

Implements #196 (closed).

doc/RTD/source/CommandLineOptions/index.rst

@@ -38,6 +38,8 @@ can be found by typing ``./swift -h``:
                                       perform black hole seeding.
     -x, --velociraptor                Run with structure finding.
     --limiter                         Run with time-step limiter.
+    --sync                            Run with time-step synchronization
+                                      of particles hit by feedback events.
 
   Control options: