Add a merge method for FilteredLogger

[uhafner]

No description provided.

[codecov-commenter]

Codecov Report

Merging #216 into master will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #216      +/-   ##
============================================
+ Coverage     88.08%   88.19%   +0.11%     
- Complexity       78       79       +1     
============================================
  Files             8        8              
  Lines           319      322       +3     
  Branches         47       47              
============================================
+ Hits            281      284       +3     
  Misses           30       30              
  Partials          8        8              

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/edu/hm/hafner/util/FilteredLog.java	100.00% <100.00%> (ø)	15.00 <1.00> (+1.00)

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