Compare mode for LTM

You can now compare seasons / date ranges across or between
athletes on the LTM charts.

This is only shown on the stack chart as we need one chart
per data series - in a similar vein to the AllPlot chart.

There are some tidy ups left to do over the next few days;

- Data table needs updating to support compare mode
- Event markers need to be shown and in the right color
- PMC curve data is slow, needs some kind of optimisation
- The tooltip is missing and needs to be put back
- Ensure the new stack frame looks correct when using a
  dark plot background (or anything other than white)
- Consider how to handle zooming when there is only one
  data series and hence only one chart for compare

src/DBAccess.cpp

@@ -78,8 +78,9 @@
 // 57  20  Jan 2014 Mark Liversedge    Added W' Expenditure for total energy spent above CP
 // 58  23  Jan 2014 Mark Liversedge    W' work rename and calculate without reference to WPrime class (speed)
 // 59  24  Jan 2014 Mark Liversedge    Added Maximum W' exp which is same as W'bal bur expressed as used not left
+// 60  05  Feb 2014 Mark Liversedge    Added Critical Power as a metric -- retreives from settings for now
 
-int DBSchemaVersion = 59;
+int DBSchemaVersion = 60;
 
 DBAccess::DBAccess(Context* context) : context(context), db(NULL)
 {