Improve export filtering for csv

[Ed McDonagh]

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.

[Ed McDonagh]

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.

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[Ed McDonagh]

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.

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