make shallow copies of track data
make shallow copies of track data

find some indices and use `array.slice` instead of building up a new
list. Still needs some algorithmic improvement. A naiev `_.sortedIndex`
against start and end coordinates won't work because the end coordinates
might not be sorted (for when a track overlaps)

refs #846 (.5)

NProfileAnalysisComputationalTool/npact,victor-lin/npact,NProfileAnalysisComputationalTool/npact,victor-lin/npact,victor-lin/npact,victor-lin/npact,NProfileAnalysisComputationalTool/npact,NProfileAnalysisComputationalTool/npact,victor-lin/npact,NProfileAnalysisComputationalTool/npact