Siemens DX filter types not imported

[Ed McDonagh]

Some Siemens DX images encode their filter data in the FilterType tag as a string with both the material and the thickness included, such as CU_0.2_MM

This is accommodated by the dx.py extractor module, but the relevant code is never reached because the line that checks if FilterMaterial is None is reached first, and the function exits.

[Ed McDonagh]

Added test file from Siemens Multix FD and test for filter information. Fails as expected. Tidied test file up too. Refs #905

→ <<cset 679bacac29fd>>

[Ed McDonagh]

Fixed import of filter information for Siemens Multix. Modified two other tests, all now pass. Refs #905

→ <<cset 71ab80582417>>

[Ed McDonagh]

Running black on the files changed in this issue (needs to be run on the rest too). Refs #905

→ <<cset bc380cfa2462>>

[Ed McDonagh]

Adding ref #905 to changes.

→ <<cset afaf49e19de9>>

[Ed McDonagh]

Merged in issue905siemensDXfilter (pull request #458)

Filter extraction code missing Multix DX filters

Fixes #905 Approved-by: David Platten

→ <<cset 1691322adf6b>>