Tracer particles

Maybe a structure like this:

```
<physics>
tracers = true

<tracers>

<tracers/initialize/sphere>
tstart = ...
origin = ...
radius = ...
nparticles = ...

<tracers/initialize/box>
tstart = ...
x1min =  ...
x1max = ...
x2min = ...
x2max = ...
nparticles = ...
```