Issue #48 required improvements to the export filtering which if applied across the board would make exports more robust.
Therefore apply the same testing for filtering content on the three csv exports.
Addresses #48. CT xlsx export now works with patient age criteria. Also improved filtering to only filter on fields that have a value. This would probably have been a better way to fix previous export errors - see #28 and #43. TODO apply new technique to csv CT, fluoro and mammo exports -- see #49 -- and add age filters to fluoro and mammo.
Fixes #48 and #49. Patient age filtering and exporting now works. Exports are now more robust rendering the modifications made for refs #28 and #43 unnecessary. Filtering before exporting is now unnecessary - see #50 for docs changes required.