FormatNXmx: open files in SWMR mode

[rjgildea]

No description provided.

[benjaminhwilliams]

LGTM. As discussed, any tests for this would be rather involved since we would need to simulate the creation of files in SWMR mode, while also trying to read them. So that might be one for the long list.

[codecov]

Codecov Report

Merging #478 (236c930) into main (42b1945) will not change coverage.
The diff coverage is 33.33%.

@@           Coverage Diff           @@
##             main     #478   +/-   ##
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  Coverage   65.70%   65.70%           
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  Files         185      185           
  Lines       17045    17045           
  Branches     2397     2397           
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  Hits        11200    11200           
  Misses       5281     5281           
  Partials      564      564           

[graeme-winter]

I may be able to come up with a test, but that should not hold up this PR

I did this for the original PIA SWMR stuff