Fix zero AHI in CSV session export when there are multiple slices.

This only affected PRS1, and it seems like the other places where
the broken Session::hours() result was used were:

- GT_SESSIONS, which seems to be unused
- settings_wavg, where it had no effect

The other uses of Settings::hours() were where there was only
a single slice, in which case it returned the right result:

- gSessionTimesChar::paint (the one still in use)
- Icon loader
- Resmed loader

Htmldocs/release_notes.html

@@ -24,6 +24,7 @@
       <li>[new] Support AVAPS in the Overview pressure chart.</li>
       <li>[fix] Fix missing bars in the Overview pressure chart for Philips Respironics devices.</li>
       <li>[fix] Add missing Philips Respironics pressure channels to CSV export.</li>
+      <li>[fix] Fix zero Philips Respironics AHI in CSV session export.</li>
       <li>[fix] Add support for the Bi-Flex lock setting on pre-DreamStation ventilators.</li>
       <li>[fix] Fix the pressure waveform scale for the BiPAP autoSV Advanced 30 (960T)</li>
       <li>[fix] Add support for rise time mode on DreamStation BiPAP devices (600X-700X).</li>

oscar/SleepLib/session.h

@@ -194,6 +194,7 @@ class Session
                     t += slice.end - slice.start;
                 }
             }
+            t = t / 3600000.0;
         }
         return t;
     }