small modification to fix bond/react to allow equal style variables as probability o

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Changes to be committed:
	modified:   doc/src/fix_bond_react.rst
	new file:   examples/USER/reaction/nylon,6-6_melt/in.large_nylon_melt_variable_probability
	modified:   src/USER-REACTION/fix_bond_react.cpp
	modified:   src/USER-REACTION/fix_bond_react.h

examples/USER/reaction/nylon,6-6_melt/in.large_nylon_melt_variable_probability

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+# 35,000 atom nylon melt example
+
+units real
+
+boundary p p p
+
+atom_style full
+
+kspace_style pppm 1.0e-4
+
+pair_style lj/class2/coul/long 8.5
+
+angle_style class2
+
+bond_style class2
+
+dihedral_style class2
+
+improper_style class2
+
+read_data large_nylon_melt.data.gz
+
+variable runsteps equal 200
+varaible prob equal step/v_runsteps
+
+velocity all create 800.0 4928459 dist gaussian
+
+molecule mol1 rxn1_stp1_unreacted.data_template
+molecule mol2 rxn1_stp1_reacted.data_template
+molecule mol3 rxn1_stp2_unreacted.data_template
+molecule mol4 rxn1_stp2_reacted.data_template
+
+thermo 50
+
+# dump 1 all xyz 100 test_vis.xyz
+
+fix myrxns all bond/react stabilization yes statted_grp .03 &
+  react rxn1 all 1 0.0 2.9 mol1 mol2 rxn1_stp1_map &
+  react rxn2 all 1 0.0 5.0 mol3 mol4 rxn1_stp2_map
+
+# stable at 800K
+fix 1 statted_grp_REACT nvt temp 800 800 100
+
+# in order to customize behavior of reacting atoms,
+# you can use the internally created 'bond_react_MASTER_group', like so:
+# fix 2 bond_react_MASTER_group temp/rescale 1 800 800 10 1
+
+thermo_style custom step temp press density f_myrxns[1] f_myrxns[2] # cumulative reaction counts
+
+# restart 100 restart1 restart2
+
+run 200
+
+# write_restart restart_longrun
+# write_data restart_longrun.data