Properly handle nulls

[bmschmidt]

Right now, any data marked as NULL in the arrow file is being plotted with a value (usually 0). Short of just manually setting things to -999, a really easy solution to unpacking the bitmasks is not clear to me.

[bmschmidt]

One possibility we've been discussing is allowing the arrow files to declare and pass a null placeholder value as a uniform to the webgl shaders. In general this would be 0, but in cases where zero is meaningful, it could be defined as anything else that happens not to occur in the data. Arrow does not care what the value behind the null bitmask is, so it's kosher to just set it to whatever.