br>
 Each line of the block describes an external field; this way you can actually have more
 than one laser (<i>e.g.</i> a "pump" and a "probe").
<br><br>
 The syntax of each line is:
<br><br>
 <tt>%TDExternalField
 <br>&nbsp;&nbsp; type | ...other descriptors...
 <br>%</tt>
<br><br>
 The first element of each line describes which kind of external field is described
 by the line: (i) an electric field (<tt>electric_field</tt>); (ii) a magnetic field
 (<tt>magnetic_field</tt>); (iii) a vector potential (<tt