Fix #425: irold not updated in electron step loop

Assign the current particle region, ir(np), to the old region index,
irold, within the ustep loop in electron transport. Previously, irold
was not updated, so the irnew ~= irold condition added to fix magnetic
field transport in vacuum caused zero-size steps to skip the region
change macro. Remove the now redundant irold = ir(np) assignment in the
non-zero step size block.

HEN_HOUSE/src/egsnrc.mortran

@@ -1366,38 +1366,27 @@ $start_new_particle;
  "Do fast step in vacuum
  IF(ustep ~= 0)
  [
- IF $EM_MACROS_ACTIVE
- [
- edep = pzero; "no energy loss in vacuum
- "transport in EMF in vacuum:"
- "only a B or and E field can be active"
- "(not both at the same time)"
- $EMFieldInVacuum;
- ]
- ELSE
- [
- "Step in vacuum
- vstep = ustep;
- tvstep = vstep;
- "( vstep is ustep truncated (possibly) by howfar
- " tvstep is the total curved path associated with vstep)
- edep = pzero; "no energy loss in vacuum
- $VACUUM-ADD-WORK-EM-FIELD;
- "additional vacuum transport in em field
- e_range = vacdst;
- $AUSCALL($TRANAUSB);
- "Transport the particle
- x(np) = x(np) + u(np)*vstep;
- y(np) = y(np) + v(np)*vstep;
- z(np) = z(np) + w(np)*vstep;
- dnear(np) = dnear(np) - vstep;
- "(dnear is distance to the nearest boundary
- " that goes along with particle stack and
- " which the user's howfar can supply (option)
- $SET-ANGLES-EM-FIELD;
- "default for $SET-ANGLES-EM-FIELD; is ; (null)
- "(allows for EM field deflection
- ] "end of EM_MACROS_ACTIVE block"
+ "Step in vacuum
+ vstep = ustep;
+ tvstep = vstep;
+ "( vstep is ustep truncated (possibly) by howfar
+ " tvstep is the total curved path associated with vstep)
+ edep = pzero; "no energy loss in vacuum
+ $VACUUM-TRANSPORT-EM-FIELD;
+ "additional vacuum transport in em field
+ e_range = vacdst;
+ $AUSCALL($TRANAUSB);
+ "Transport the particle
+ x(np) = x(np) + u(np)*vstep;
+ y(np) = y(np) + v(np)*vstep;
+ z(np) = z(np) + w(np)*vstep;
+ dnear(np) = dnear(np) - vstep;
+ "(dnear is distance to the nearest boundary
+ " that goes along with particle stack and
+ " which the user's howfar can supply (option)
+ $SET-ANGLES-EM-FIELD;
+ "default for $SET-ANGLES-EM-FIELD; is ; (null)
+ "(allows for EM field deflection
  ] "end of vacuum step
 
  IF(irnew ~= irold) [ $electron_region_change; ];