Reporting issues from mean_intensity

Fix for #91

@kslong - have added counter which counts number of times the two situations we encounter in mean_intensity occur. Then  report with a log statement in wind_updates which looks like this

```C  
/* Added this system which counts number of times two situations occur (See #91)
     We only report these every 100,000 times (one can typically get ) */
  Log ("wind_update: note, errors from mean intensity can be high in a working model\n");
  Log ("wind_update: can be a problem with photon numbers if errors from spectral_estimators and low photon number warnings\n");
  Log ("wind_update: mean_intensity: %i x10^6 occurrences, this cycle, this thread of 'no model exists in a band'\n",
  	  	      nerr_no_Jmodel / 1e6);
  Log ("wind_update: mean intensity: %i x10^6 occurrences, this cycle, this thread of 'photon freq is outside frequency range of spectral model'\n",
  	  	     nerr_Jmodel_wrong_freq / 1e6);
```

Few other cosmetic changes too. If you're happy I'll merge.