Add maximum complexity of all methods as a metric

[uhafner]

Summing up the complexities of all methods might be not a good metric for a project. It makes more sense to look at the maximum complexity of all methods.

[codecov]

Codecov Report

Merging #5 (cf859d3) into main (55e9852) will decrease coverage by 0.17%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##               main       #5      +/-   ##
============================================
- Coverage     85.40%   85.23%   -0.17%     
- Complexity      476      478       +2     
============================================
  Files            23       23              
  Lines          1302     1321      +19     
  Branches        187      193       +6     
============================================
+ Hits           1112     1126      +14     
- Misses          145      150       +5     
  Partials         45       45              

Impacted Files	Coverage Δ
src/main/java/edu/hm/hafner/coverage/Value.java	29.03% <ø> (ø)
src/main/java/edu/hm/hafner/coverage/Metric.java	89.18% <60.00%> (+0.66%)	⬆️
src/main/java/edu/hm/hafner/coverage/Node.java	80.99% <100.00%> (-1.34%)	⬇️

... and 2 files with indirect coverage changes

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