libpsdxx [FIX]: rename variables (frequency interval not time interval)

src/libs/libpsdxx/function_lin_log_sampling.cc

@@ -47,12 +47,12 @@ namespace psd {
     PSDXX_assert(s.size()>1, "input series must contain at least 2 values");
     PSDXX_assert(s.size()==flin.size(),
                  "series of frequencies does not match data series");
-    double dt1=flin(flin.f()+1)-flin(flin.f());
-    double dt2=flin(flin.l())-flin(flin.l()-1);
-    PSDXX_assert(std::abs((dt2/dt1)-1.)<1.e-8,
+    double df1=flin(flin.f()+1)-flin(flin.f());
+    double df2=flin(flin.l())-flin(flin.l()-1);
+    PSDXX_assert(std::abs((df2/df1)-1.)<1.e-8,
                  "input frequency scale is not linear");
     TDISeries::Tcoc data;
-    data.interval=dt1;
+    data.interval=df1;
     data.data=s;
     TDseries retval=log_sampling(data, flog);
     return(retval);