Prevent exploratory heatmaps that are too large from rendering

[rabstejnek]

The computed matrixDataset is the first hurdle when it comes to rendering large heatmaps: the fix here is to split the HeatmapDatastore constructor so that a few properties set in the constructor are used to determine if the heatmap is too large, while the initialize method continues the computation intensive initialization.

Note:

• It still hangs a little to perform the basic calculations to determine table size when the table is REALLY large, but it should not freeze the page.

[shapiromatron]

Looks great to me, but I made a few changes in review that I was hoping you could take a look at.

• I tried to approach this more as a guarding method. Previously, we rendered a no dataset warning before, and I just added this below that
• I moved all the logic into the store
• I also moved the hasDataset and settingsHash logic into the store instead of the view too

One bonus change, I renamed "X fields" and "Y fields" to "Columns" and "Rows", because that seems much more intuitive given that this is really just a big 2d table.

@rabstejnek if you're ok with my revisions, feel free to merge, else we can discuss more!