Implement time offsets for uniformly sampled data

[anthonytw]

For uniformly sampled data the time channel adds unnecessary bulk. This can be fixed by implementing a meta time column that includes a set of index to time and frequency mappings, e.g.:

{
   ...
   "columns": {
      ...
      "time": {
         "type": "time-mapping",
         "offsets": [
            { "index": 0, "offset": 0, "freq": 100.0 },
            { "index": 1234, "offset": 442.2, "freq": 20.0 }
         ]
      }
      ...
   }
   ...
}

The offset is wrt the time offset (either global or local to the dataset), and the multiple entries support gaps and varying sample frequencies.