Unit and uncertainty handling when reading `pars`

[theHenks]

This PR enables features to read and write pars with units and uncertainties from json files. In particular, the following functions are provided:

• uparse(pd::PropDict) and ustrip(pd::PropDict): Parse units and strip units from a PropdDict. The units have to be stored in the :unit field while the value and error have to be in the :val and :err field. The parsing remove the :val key completely if the there is only the :val and :unit field present. The :unit field has to be able to be parsed by Unitful. Ustrip performs the opposite operation
• measurement(pd::PropDict) and mstrip(pd::PropDict): Parse uncerainties from the :err field into the variable. The parsing remove the :val key completely if there is only the :val and :err key present.
• readlprops and writelprops: New read and write function for LEGEND pars which automatically reads and writes units and uncertainties. Example input:

  PropDict with 2 entries:
  :P00664A => PropDict(:n_tau=>5269, :sigma=>PropDict(:unit=>"μs", :val=>5.68755, :err=>0.0770621), :tau=>PropDict(:unit=>"μs", :val=>522.829, :err=>0.0855794))
  :V02166B => PropDict(:n_tau=>22106, :sigma=>PropDict(:unit=>"μs", :val=>1.88776, :err=>0.0121664), :tau=>PropDict(:unit=>"μs", :val=>431.417, :err=>0.0137728))

  Example output:

  PropDict with 2 entries:
  :P00664A => PropDict(:n_tau=>5269, :sigma=>5.688±0.077 μs, :tau=>522.829±0.086 μs)
  :V02166B => PropDict(:n_tau=>22106, :sigma=>1.888±0.012 μs, :tau=>431.417±0.014 μs)

[codecov[bot]]

Codecov Report

Attention: 36 lines in your changes are missing coverage. Please review.

Comparison is base (59e5b50) 48.96% compared to head (c9fd111) 48.37%. Report is 9 commits behind head on main.

:exclamation: Current head c9fd111 differs from pull request most recent head f4ce721. Consider uploading reports for the commit f4ce721 to get more accurate results

Files	Patch %	Lines
src/filekey.jl	0.00%	17 Missing :warning:
src/lprops.jl	55.88%	15 Missing :warning:
src/dataprod_config.jl	0.00%	2 Missing :warning:
src/legend_data.jl	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #26 +/- ## ========================================== - Coverage 48.96% 48.37% -0.59% ========================================== Files 15 16 +1 Lines 966 1017 +51 ========================================== + Hits 473 492 +19 - Misses 493 525 +32 ```

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