Set grainColumnName as optional

[diazd2]

Setting the grainColumnName as optional in the PhoenixChartDataColumn interface as it was causing errors when initializing ChartData objects.

The documentation has multiple spots where objects using this interface's structure are shown but the grainColumnName field was missing. A customer run into errors while creating charts, following the steps in the docs.

e.g. https://domoapps.github.io/domo-phoenix/#/domo-phoenix/framework/react where type, name, and mapping are correctly listed, but not grainColumnName.

Since grainColumnName seems to be linked to dateGrain, we're thinking it can be marked as optional instead.

If it is actually meant to be required, we'll need to update the docs.

-- Bug reported by @WaldoMarais