Proper default for FormatXTC's delta_k

[phyy-nx]

No description provided.

[phyy-nx]

This will fix an error in XFEL CI, psana version, that came from #484, so I'll merge this after the checks pass. The error was that delta_k needs to have a default of 0 instead of None.

[codecov]

Codecov Report

Merging #490 (ca972f7) into main (8d9a462) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #490   +/-   ##
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  Coverage   40.60%   40.60%           
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  Files         177      177           
  Lines       15564    15564           
  Branches     2779     2779           
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  Hits         6320     6320           
  Misses       8703     8703           
  Partials      541      541