Fix #125: Sync DOSXYZnrc source 20 with IAEA phsp

Monitor unit (MU) index from IAEA phase space sources was not read in by
DOSXYZnrc. The issue was twofold:

1. For all iaea phase space sources, regardless of whether MU was stored
   or not: iaea_i_muidx, the index at which MU values are stored in a
   floating point array, was automatically reset to -99 (i.e., no MU
   values stored) due to erroneous invoking of the $INIT_PHSP_COUNTERS
   macro at the beginning of subroutine srcinit.

2. For iaea phase space sources storing one or more floating point
   values (e.g., ZLAST) but not MU: The DOSXYZnrc flag, i_mu_phsp,
   indicating MU is stored in this data, was erroneously set to 1. This
   resulted in DOSXYZnrc failing to set a value of MU for each incident
   particle, defaulting to a value of 0 instead. The erroneous setting
   of i_mu_phsp was due to a bug in the macro $IAEA_READ_PHSP_HEADER
   (in $HEN_HOUSE/utils/iaea_phsp_macros.mortran).

HEN_HOUSE/user_codes/dosxyznrc/srcxyznrc.mortran

@@ -2397,8 +2397,6 @@ $REAL weight,
       cosphi,sinphi;
 $INTEGER i;
 
-$INIT_PHSP_COUNTERS;
-
 OUTPUT61;(' ');
 
 "------------------------------------------------------------------------"

HEN_HOUSE/utils/iaea_phsp_macros.mortran

@@ -253,7 +253,7 @@ ELSE[
 "we assume this is the first float after zlast, if zlast is scored,"
 "and has generic user type = 0"
 iaea_i_muidx=MAX(1,iaea_i_zlast+1);
-IF(iaea_n_extra_floats=0 | iaea_extra_float_types(iaea_i_muidx)~=0) [
+IF(iaea_i_muidx>iaea_n_extra_floats | iaea_extra_float_types(iaea_i_muidx)~=0) [
   iaea_i_muidx=-99;"reset this to no scoring"
   {P8}=0;
 ]