Add multi-track feature

[kaitmore]

The api should be able to handle a multi-dimensional array of data points and create a "track" for each top level array, e.g.

  [
    [ // track 1
      [1536969666906, 1],
      [1546969674206, 2],
      [1556966695555, 4.3],
      [1566959697655, 2.3],
      [1576669693906, 3]
    ],
    [ // track 2
      [1536969666906, 2],
      [1546969674206, 1],
      [1556966695555, 2.3],
      [1566959697655, 3.3],
      [1576669693906, 1]
    ]
  ];