Store mean filter thickness values in exports

[Ed McDonagh]

Flat filters are normally recorded with a min and max of +/- a small amount from the nominal. It is normally the nominal that would be useful to know, so we should export the mean instead.

Not sure what to do about real wedge filters - I have never seen one recorded in DICOM though.

[Ed McDonagh]

Already done for CSV DX exports in 5709e01

[Ed McDonagh]

Factored out filter and filter_thickness value population to replace the three occurancers with one. Refs #454 for xlsx, already fixed for csv. Also fixes #430 for all three occurances in csv and xlsx. Test needs updating.

→ <<cset 28c86f8aad16>>

[Ed McDonagh]

Updated test for ref #454.

→ <<cset abb6f711c0f8>>

[Ed McDonagh]

Merging issue430-442errorcatching back into develop. Fixes #442, refs #430,#449,#454,#458

→ <<cset 4089c23ddffc>>

[Ed McDonagh]

Adding ref #454 to changes

→ <<cset 08cee7d5c461>>