MRG: allow get/set record.filename

[bluegenes]

I need to be able to access a record's filename from branchwater to reproduce full gather results. I think this should allow that.

any thoughts/objections, @luizirber? Should I make it get only?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.55%. Comparing base (08fe5ba) to head (f640c1e).

Additional details and impacted files

@@           Coverage Diff           @@
##           latest    #3121   +/-   ##
=======================================
  Coverage   86.55%   86.55%           
=======================================
  Files         136      136           
  Lines       15809    15809           
  Branches     2713     2713           
=======================================
  Hits        13683    13683           
  Misses       1816     1816           
  Partials      310      310           

Flag	Coverage Δ
hypothesis-py	25.36% <ø> (ø)
python	92.32% <ø> (ø)
rust	61.47% <ø> (ø)

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