Tracking with wrong sign for R_ref gives NaN with no message

Test program:
```
using BeamTracking
using Beamlines

@eles begin
q = Drift(L = 1, tracking_method = Exact())
end

bb = Beamline([q], R_ref = -1e7, species_ref = Species("electron"))

v1 = [0.0 0.0 0.0 0.0 0.0 0.0]

b1 = Bunch(deepcopy(v1), species=Species("electron"), R_ref = -1e7)
track!(b1, q)
println(b1.coords.v)

b2 = Bunch(deepcopy(v1), species=Species("electron"), R_ref = 1e7)
track!(b2, q)
println(b2.coords.v)
```

Output is:
```
julia> include("local/z.jl")
[0.0 0.0 0.0 0.0 0.0 0.0]
[0.0 0.0 0.0 0.0 NaN 0.0]
```