Add column function to dsv.parse

[pietersv]

This adds an optional function to transform column names to dsv.parse(...), comparable to the row accessor added in #1107,

The motivation is that input datafiles may have column names...

• with arbitrary case generated from other packages that are case insensitive like SAS or SPSS, requiring conversion to some defined case in Javascript
• with duplicate / missing column names, that need to be deduped, cleaned up, or invented, (e.g. see rules for "Variable Names" in Stat/Transfer

Tests are added that cover these two examples:

var rows = d3.csv.parse(csv, null, function(name) { return name.toLocaleLowerCase(); })
var rows = d3.csv.parse(csv, null, function(name, i) { return "var"+i; })

http://stackoverflow.com/questions/26998476/right-way-to-modify-d3-csv-to-lower-case-column-names

[mbostock]

I feel the existing row accessor function is sufficient for renaming columns. For example, to lower-case column names:

var rows = d3.csv.parse(csv, function(d) {
  return {
    a: d.A,
    b: d.B,
    c: d.C
  };
});

It’s easiest if you know the column names, but you could, I suppose, write the code generically (at the cost of performance and complexity). Or you could replicate the approach taken by dsv.parse and wrap dsv.parseRows.

I agree that having a dedicated function for remapping columns is simpler, but I think the existing API is sufficient and we should endeavor to keep the API small.