BUG: Fix the param columns being incorrectly written to the .hdf5 file

[BvB93]

cc @RobertaPascazio

Note that this will change the output of FOX.from_hdf5(filename, datasets="param"),
as the latter will now return a list of dataframes.

[codecov]

Codecov Report

Merging #266 (fa92490) into master (933242f) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #266      +/-   ##
==========================================
- Coverage   75.26%   75.24%   -0.02%     
==========================================
  Files          66       66              
  Lines        7054     7058       +4     
  Branches     1355     1356       +1     
==========================================
+ Hits         5309     5311       +2     
  Misses       1408     1408              
- Partials      337      339       +2     

Impacted Files	Coverage Δ
FOX/io/hdf5_utils.py	80.52% <100.00%> (+0.11%)	⬆️
FOX/testing_utils.py	71.33% <100.00%> (-0.97%)	⬇️
FOX/io/read_psf.py	80.11% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 933242f...fa92490. Read the comment docs.