Crash when only providing electric field and not magnetic field

[rtownson]

A segmentation fault occurs when only an electric field is specified:

Electric Field = 1000,0,0
EM ESTEPE = 0.2

the error is resolved when you provide a zero magnetic field:

Magnetic Field = 0,0,0
Electric Field = 1000,0,0
EM ESTEPE = 0.2

So it seems that we need to add a default setting for magnetic fields of 0,0,0. I haven't looked into the code yet, and only tested this in egs++.