Use memmap=False instead of copy_arrays=False when opening asdf files

[wtbarnes]

Fixes #338

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.96%. Comparing base (8b9ef1f) to head (b34cd19).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #339      +/-   ##
==========================================
+ Coverage   90.55%   90.96%   +0.40%     
==========================================
  Files          41       41              
  Lines        3400     3399       -1     
==========================================
+ Hits         3079     3092      +13     
+ Misses        321      307      -14     

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