[FEATURE] (issue7_refract): make offset dependent scaling more robust

- With the previous commits the average scaled amplitude in a given offset
  range is made the standard reference for amplitude scaling in modes 2 and 3.
- This commit makes sure that the scaling exponent will never become negative.

A negative scaling exponent would result in a singularity of scaled amplitudes
at zero offset. Since amplitudes always are expected to decay with offset,
negative exponents are not required for amplitude scaling.

src/ts/refract/sub/refract_cmdopt.f

@@ -267,7 +267,7 @@ c ============================
 c
       if (optset(24)) plflag_linestyle=opt_Lcycle
       if (optset(29)) plflag_color=opt_Ccycle
-      if (optset(46)) plpar_expo=opt_Sexp
+      if (optset(46)) plpar_expo=max(0.,opt_Sexp)
       plpar_forceminoff=optset(53)
       if (plpar_forceminoff) plpar_minoff=opt_Sminoff
       if (optset(54)) plpar_radius=opt_Sradius

src/ts/refract/sub/refract_loopaction.f

@@ -381,7 +381,7 @@ c
             flag_replot=.true.
           elseif (plpar_pmmode.eq.3) then
             if (verbose) print *,'reduce scaling exponent...'
-            plpar_expo=plpar_expo-0.08
+            plpar_expo=max(0.,plpar_expo-0.08)
             call mpcfactors
             flag_replot=.true.
           else

src/ts/refract/sub/refract_usage.f

@@ -186,7 +186,7 @@ c
       print *,' '
       print *,'-Sx x1,x2      display offset range [x1,x2]'
       print *,'-St t1,t2      display time range [t1,t2]'
-      print *,'-Se exp        set scaling exponent'
+      print *,'-Se exp        set scaling exponent (must not be negative)'
       print *,'-Sa lev        set scaling amplitude level'
       print *,'-Sc lev        set scaling clipping level'
       print *,'-Sm mode       set scaling mode'