Add tach sensors to each fan enclosure

Create a tach sensor instance for each sensor listed within a fan
enclosure that uses 'tach' based presence detection. Each tach sensor
has a pointer to the fan it's associated with and is added to the list
of sensors for that fan enclosure.

Change-Id: I9a83ec52d1a5d01e39702e185336a09edeb4d158
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
diff --git a/tach_sensor.cpp b/tach_sensor.cpp
index 1325365..987e390 100644
--- a/tach_sensor.cpp
+++ b/tach_sensor.cpp
@@ -10,7 +10,7 @@
 
 bool TachSensor::isPresent()
 {
-    return false;
+    return (tach != 0);
 }
 
 } // namespace presence