Fix #257: rest mass used before it is set in HATCH

Initialize prm (precise rest mass) in the COMIN/USEFUL declaration to
fix a critical bug introduced in commit 1eaf898. The bug affects
applications where prm is used before HATCH is called, so at least the
following ones:

HEN_HOUSE/omega/beamnrc/beamnrc.mortran
HEN_HOUSE/user_codes/cavrznrc/cavrznrc.mortran
HEN_HOUSE/user_codes/cavsphnrc/cavsphnrc.mortran
HEN_HOUSE/user_codes/dosxyznrc/dosxyznrc.mortran
HEN_HOUSE/user_codes/sprrznrc/sprrznrc.mortran

In DOSXYZnrc, for example, it affects monoenergetic sources, resulting
in the energy of incident electrons being increased by the value of the
electron rest mass.

HEN_HOUSE/src/egs_utilities.mortran

@@ -940,9 +940,6 @@ kmpi = 12; kmpo = 8; dunit = 1;
 rng_seed = 999999;
 latchi = 0;
 
-" The rest mass value is as recommended by CODATA 2014
-" http://physics.nist.gov/cgi-bin/cuu/Value?mec2mev
-rm = 0.5109989461;
 rmt2 = 2*rm; rmsq = rm*rm;
 
 pi = 4*datan(1d0); twopi = 2*pi; pi5d2 = 2.5*pi;

HEN_HOUSE/src/egsnrc.macros

@@ -1067,6 +1067,9 @@ REPLACE {;COMIN/USEFUL/;} WITH
  $REAL RM; "electron mass in MeV"
  $INTEGER MEDIUM, "medium index of current region"
  MEDOLD; "medium index of previous region"
+ " The rest mass value is as recommended by CODATA 2014"
+ " http://physics.nist.gov/cgi-bin/cuu/Value?mec2mev"
+ DATA RM,PRM,PRMT2,PZERO/0.5109989461,0.5109989461,1.0219978922,0.D0/;
 }
 
 "------------------------------------------------------------------"

HEN_HOUSE/src/egsnrc.mortran

@@ -1843,11 +1843,6 @@ $INTEGER lnblnk1;" in house lnblnk function becuase not all compilers"
 DATA MDLABL/$S' MEDIUM='/,LMDL/8/,LMDN/24/,DUNITO/1./;
 DATA I1ST/1/,NSINSS/37/,MXSINC/$MXSINC/,ISTEST/0/,NRNA/1000/;
 
-" SET UP ENERGY PRECISION VARIABLES"
-PRM=RM; "PRECISE REST MASS"
-PRMT2=2.D0*PRM; "TWICE THE PRECISION REST MASS"
-PZERO=0.0D0; "PRECISE ZERO"
-
 $INIT-PEGS4-VARIABLES;
 
 " FORMAT STATEMENTS USED MULTIPLE TIMES IN SETUP"