Computed columns in charts, view data, etc

[davidrg]

Support computed columns in:

• Charts
• View Data
• Maybe export

Columns would be computed using either JavaScript or a custom expression language. JavaScript would probably be best - especially if additional stuff could be easily loaded into the script context allowing things like sunrise/sunset or THSW to be calculated in this way.

UI would be an extra tab in the chart/view data/etc options dialog with a UI for adding from a list of available columns. Would need a way of defining these columns somehow too - possibly this could just be a js file or maybe something fancier.

Columns would be computed outside of the data source layer unless #41 has been done (in which case ideally this would be a feature of the data source layer).