Update code style, line length, member order, etc.

[mdabros]

Part of 2019 sommer cleaning to get the code base cleaned and more up to date. This pull request contains mainly code style changes. This includes:

• Line lengths around 100 chars (unless specific circumstances, like constructor checks, and expected test results)
• Remove and order usings.
• Class member ordering to follow standard guidelines.
• Use expression bodied methods where applicable.

The pull request also includes code changes to avoid duplicated code in a few places.

A few more exotic metrics has also been deleted:

• DiscreteTargetMeanErrorRegressionMetric
• RocAucRegressionMetric

If anybody uses these, let me know, and I can readd them.