[perf model] Move the interval stat class to it's own file.
[perf model] Move the interval stat class to it's own file.

nkawahara/Graphite,mit-carbon/Graphite,8l/Graphite,fhijaz/Graphite,8l/Graphite,mit-carbon/Graphite-Cycle-Level,victorisildur/Graphite,nkawahara/Graphite,victorisildur/Graphite,8l/Graphite,nkawahara/Graphite,fhijaz/Graphite,fhijaz/Graphite,nkawahara/Graphite,mit-carbon/Graphite,fhijaz/Graphite,mit-carbon/Graphite-Cycle-Level,victorisildur/Graphite,mit-carbon/Graphite-Cycle-Level,mit-carbon/Graphite,mit-carbon/Graphite,mit-carbon/Graphite-Cycle-Level,victorisildur/Graphite,8l/Graphite