Add twitter link that explains use of dot product

logger.cpp

@@ -39,6 +39,7 @@ namespace sew {
       else         log_line << (w->was_energy_dissipated_since_last_logged_to_screen ? " <E" : "   ");
     }
     if (dv_logging) {
+      // https://twitter.com/akshay_pachaar/status/1782387821493584011
       log_line << " d⋅v " << std::setw( 4) << std::fixed << std::setprecision(1)
                << w->dist_vel_dot_prod    // -1 = approaching, 1 = leaving
                << std::scientific;

particle.cpp

@@ -356,7 +356,7 @@ void Particle::ConsiderLoggingToFile(int count) {
     new_dt = a_->long_dt + ((a_->short_dt - kLongDt) * inverse_exponential);
     percent_energy_dissipated = 0;
     was_energy_dissipated = false;
-    // Dissipate energy when heading away.
+    // Dissipate energy when heading away.  https://twitter.com/akshay_pachaar/status/1782387805597164016
     if (dist_vel_dot_prod >= 0
      && orig_vel_dot_prod >= 0
      && dist_mag_closest > (kCloseToTrouble*16)) {

particle.h

@@ -94,9 +94,10 @@ class Particle {
   SFloat fast_fraction = 0;  // Percent we are between long dt and short dt.
 
   SFloat orig_vel_dot_prod;  // Positive = moving away from origin.  Negative = moving towards origin.
+  // https://twitter.com/akshay_pachaar/status/1782387821493584011
   // Determine if electron is coming or going relative to closest proton.
   // For proton indicates if coming or going relative to center.
-  SFloat       dist_vel_dot_prod;
+  SFloat       dist_vel_dot_prod;     // -1 = approaching, 1 = leaving
   // Due to energy gain problem, sometimes dissipate energy when moving away from center of atom.
   bool was_energy_dissipated = false;
   bool was_energy_dissipated_since_last_logged_to_screen = false;