Primarily to make override() a base class function that can be
used for any metric rather than expecting each metric to provide
a local version.
Also, add explicit notion of "average" vs "total" ride metrics, as
it will let us improve how the metrics DB handles averages later.
A user configurable chart for showing ride metrics and
other calculated values over time.
* Uses SQLITE database to store metrics
* Supports any metric available from the metric factory
* Adds new MaxHr, VI, Peak Power and Time In Zone metric
* Also includes LTS/STS/SB for PM charting
* Aggregates in days, weeks, months or years
* Reads and Updates seasons.xml
* Adds cycles and adhoc date ranges in seasons.xml
* Date ranges can be selected on the plot with shift-left click
* Allows users to customise preferences for color, symbols et al
* Allows user to customise metric names and unit names
* Supports smooth curves and topN highlighting
* Has a linear regress trend line function
* Allows users to save charts to charts.xml
* A default charts.xml is built-in
* A chart manager to import/export/rename/delete charts etc
* Provides a tooltip to provide basic datapoint information
* Performance Manager adjusted to use the MetricDB
* User configurable setting for SB calculation (today/tomorrow)
