ExternalCellRefDetector returns a set of UnresolvedRef
Fixing filename typo.
Adding fast detect function to ExternalCellRefDetector.
fix: if the cell can not be evaluated then try to return the cached value. this is particularly useful with erternal refs in xlsx files as they are currently not implemted in poi
implemented externalReferences on XlsxDataSource
Adding initial work on SpreadsheetController.

Some key parts are missing implementations.
Adding routes and some tests for SpreadsheetController.
Changing ResolvedRef to allow for missing datasources.
refactored and implemented externalReference on XlsDataSource
Initial work on cell resolver.

Still a work in process, but all (very incomplete) tests are green.
Implementing name-based ref resolution.
Adding simple implementation of ExternalCellRefDetector.
Adding tests for detector.
Raising futures into the controller, as lower level is unavoidably synchronous.
Adding external cells update button to UI.
Switching from GET to HEAD for external cell reference check.
Adding more tests for SpreadsheetController.

Includes a pending test for actual replacement, which is not yet implemented.
Adding more tests for SpreadsheetDataSourceExternalCellRefDetector.
implemented updateExternalReferences on the SpreadsheetDataSource
Connecting together components.

Replacement of cells doesn't seem to affect charts for some reason.