Use child property tree

[Rombur]

Use child property tree to group input parameters.

Relates to #37

[codecov-io]

Codecov Report

Merging #48 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #48      +/-   ##
=========================================
+ Coverage   88.78%   88.8%   +0.02%     
=========================================
  Files          19      19              
  Lines        1106    1108       +2     
=========================================
+ Hits          982     984       +2     
  Misses        124     124

Impacted Files	Coverage Δ
tests/test_restriction_matrix.cc	100% <100%> (ø)	⬆️
tests/test_hierarchy.cc	82.23% <100%> (ø)	⬆️
include/mfmg/dealii_adapters.hpp	91.3% <100%> (+0.39%)	⬆️
include/mfmg/dealii_operator.templates.hpp	57.3% <100%> (ø)	⬆️

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