Multiple Headers Support (Plugin out of date)

Attempting to have 2 header rows on the latest release. Using the `grouped-header` plugin throws an error when calling `grid.installPlugins(require('grouped-header'))`.

```
Uncaught TypeError: Cannot read property 'getActiveColumn' of null
at Object.mixInTo (:.../bundle.js:1280)
at Constructor. (fin-hypergrid.github.io/core/2.1.2/build/fin-hypergrid.js:4198)
at Array.forEach ()
at Constructor.installPlugins (fin-hypergrid.github.io/core/2.1.2/build/fin-hypergrid.js:4144)
...
at Object.delegate (bundle.js:41356)
at bundle.js:49135
```

Attempting to do this by just having the secondary header as the first data row and fixing it in place with `fixedRowCount: 1` works, but at that point the leftmost index column is all off-by-one since the header counts as a data row.

Reproduce the failure by installing the grouped-header plugin with grid.installPlugins(require('grouped-header')) against core 2.1.2, then follow the stack trace through bundle.js and build/fin-hypergrid.js. Done means multiple header rows install without the getActiveColumn error and the leftmost index column remains correctly aligned when a header is fixed.

Written by the indexing model from the issue text.