fix: dict classes should produce instances of themselves

[APN-Pucky]

LHEInit and LHEProcInfo inherit from dict. When parsing a string to either of them with fromstring it should return the corresponding class not a generic dict. They should have the same __init__ function as a dict for convenience.

* `LHEInit` and `LHEProcInfo` inherit from `dict`. When parsing a string
  to either of them with `fromstring` it should return the corresponding
  class not a generic dict. They should have the same `__init__` function
  as a `dict` for convenience.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.91%. Comparing base (aefb63b) to head (c1d61a2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #232   +/-   ##
=======================================
  Coverage   91.91%   91.91%           
=======================================
  Files           2        2           
  Lines         235      235           
  Branches       54       54           
=======================================
  Hits          216      216           
  Misses         16       16           
  Partials        3        3           

Flag	Coverage Δ
unittests-3.10	91.48% <100.00%> (ø)
unittests-3.8	91.48% <100.00%> (ø)

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[matthewfeickert]

Thanks @APN-Pucky. This LGTM and I'd like this to go in before PR #233 so that things can be more atomic.

Can you please write a summary in the PR body of why this is needed, so that this can also get included in the squash and merge commit message?

[APN-Pucky]

Can you please write a summary in the PR body of why this is needed, so that this can also get included in the squash and merge commit message?

Done