Allow Timeframe to be used for filtering in Worksheet fields

As part of our maturity review we want to quickly understand if a particular workspace has planned their Initiatives and Goals in the current Timeframe (e.g. this year). To do this we'd like to create a worksheet on the workspace object, but the formula is not currently supported.

count(self.goals[self.epoch == "2023"])

returns error: 'Path self.epoch cannot be used in a condition'