Let DataLoader default setters overwrite (not append) + lgdo.Struct.int_dtype

[gracesong312]

1. Adds the key_dtype to the Struct attributes so that we can use non-strings as keys in Struct. Converts to/from string when writing to/from disk. Also returns the output of dl.load(merge_files=False) as a Struct now that ints can be used as keys.
2. Changes the default behavior of DataLoader setter methods to overwrite existing parameters, but adds an append argument in case appending is the desired behavior.

Addresses #492 first point

[codecov]

Codecov Report

Patch coverage: 81.81% and project coverage change: +0.04 🎉

Comparison is base (f4f1940) 48.42% compared to head (2cf4cc4) 48.47%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #493      +/-   ##
==========================================
+ Coverage   48.42%   48.47%   +0.04%     
==========================================
  Files         104      104              
  Lines       12461    12498      +37     
==========================================
+ Hits         6034     6058      +24     
- Misses       6427     6440      +13     

Impacted Files	Coverage Δ
src/pygama/lgdo/lh5_store.py	86.04% <66.66%> (-0.20%)	⬇️
src/pygama/flow/data_loader.py	77.01% <83.33%> (+0.52%)	⬆️
src/pygama/lgdo/struct.py	100.00% <100.00%> (ø)
src/pygama/lgdo/table.py	92.50% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[gipert]

Could you add a test of the append=False functionality? Something that makes sure that a second call to a setter + loading does not fail (i.e. does not require a call to .reset())/