Support annotation of aggregation intervals

When the data set is already aggregated, metadata should be able to specify the width of intervals used for aggregation.

For example, the data used as input in this [D3 Heatmap example](http://bl.ocks.org/mbostock/3202354) has the intervals `yStep` and `xStep` inside the visualization code. These intervals should be encoded somehow in the metadata for each column.

The issue names no files or tests; begin with the metadata specification and parsing library described by the repository, and inspect how column metadata is currently represented. Done means the format and parser can represent interval widths for aggregated columns, with the D3 Heatmap example covered.

Written by the indexing model from the issue text.