change time type from double to int64_t

change time type from double to int64_t

* grapher/Graph.hxx (Graph): Change variables holding the time limits
  of the displayed graph from double to int64_t.
* grapher/Graph.cxx (Graph::draw): Do calculations of time differences
  using int64_t.
  (Graph::getExtents, Graph::setExtents): Change left and right
  arguments to int64_t.
* grapher/GraphData.hxx (GraphDataBase): Change time type to int64_t.
  (GraphDataBase::elementAsString): New function.
  (GraphData::elementAsString): Implementation of that function.
* grapher/StapParser.cxx (parseData): Parse time values from the stap
  script as 64 bit values.