Fix #413: limit vacuum step size in magnetic field

Adjust the charged particle step routine for transport in vacuum when
there is an electromagnetic field (emf). The existing logic failed
because the emf step size restriction was not taken into account before
the howfar call. Hence the howfar method tracked the particle in a
single straight step across the entire vacuum region, causing erroneous
region changes and discards.

Also check if the particle in fact entered a new region before calling
the macro to change the region, since charged particle may take many
steps through a vacuum region when there is a field.

Remove redundant step size restriction code in the vacuum emf transport
macro, now that the field step size restriction in vacuum has been moved
ahead of howfar in the electr subroutine.

HEN_HOUSE/src/egsnrc.mortran

@@ -1111,6 +1111,10 @@ $start_new_particle;
  " direction of motion at the start of the step
  " Note that tustep and ustep are modified below.
  " The above provide defaults.)
+
+ " EM field step size restriction in vacuum
+ $SET-TUSTEP-EM-FIELD;
+ ustep = tustep;
  ]
  ELSE
  [
@@ -1383,7 +1387,7 @@ $start_new_particle;
  "(allows for EM field deflection
  ] "end of vacuum step
 
- $electron_region_change;
+ IF(irnew ~= irold) [ $electron_region_change; ];
 
  IF(ustep ~= 0) [$AUSCALL($TRANAUSA);]
  IF(eie <= ecut(irl)) [go to :ECUT-DISCARD:;]

HEN_HOUSE/src/emf_macros.mortran

@@ -88,7 +88,7 @@ APPEND {;
  ufx,ufy,ufz,
  bdotub,bsqrd,
  ufxpar,ufypar,ufzpar,
- ufxprp,ufyprp,ufzprp,ustepold;
+ ufxprp,ufyprp,ufzprp;
 } TO {$DEFINE-LOCAL-VARIABLES-ELECTR;};
 
 
@@ -291,14 +291,6 @@ IF(MEDIUM.EQ.0)[
 "$SET-TVSTEP-EM-FIELD;--this is for higher order corrections"
 "do first order corrections for now--BW"
 IF(ExIN~=0.0 | EyIN~=0.0 | EzIN~=0.0 | BxIN~=0.0 | ByIN~=0.0 | BzIN~=0)[
-USTEPOLD=USTEP;
-IF(USTEP>smaxir(irl)) USTEP=smaxir(irl);
-TUSTEP=USTEP;
-$SET-TUSTEP-EM-FIELD;
-TVSTEP=TUSTEP;
-VSTEP=TVSTEP;
-USTEP=VSTEP;
-IF(USTEP<USTEPOLD) irnew=irl;
 DE=0.0;
 $ADD-WORK-EM-FIELD;
 PEIE=PEIE-DE;EIE=PEIE;E(NP)=PEIE;