Data function flow should capture warnings from DF code

[bbassett-tibco]

Currently, data functions that emit a warning in Spotfire do so via standard error and is only displayed to the user when data function debugging is turned on. The data function flow in the spotfire.data_function module should capture warnings from data function code execution (and presented as part of the AnalyticResult object), but only if opted in to by the caller of AnalyticSpec methods (to preserve the older behavior for use with older Spotfire releases).