monitor: Change Fan/Sensor def tuples to structs

The tuples were big and hard to read/use.  Change them to structs so the
members are always named.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I79826563faf44636b251e614f45ff86f1e02c607
diff --git a/monitor/system.cpp b/monitor/system.cpp
index 0fc2c44..4a33bb5 100644
--- a/monitor/system.cpp
+++ b/monitor/system.cpp
@@ -271,7 +271,7 @@
     for (const auto& fanDef : fanDefs)
     {
         // Check if a condition exists on the fan
-        auto condition = std::get<conditionField>(fanDef);
+        auto condition = fanDef.condition;
         if (condition)
         {
             // Condition exists, skip adding fan if it fails