[!505] Feature/cleanup newton

Merge branch 'feature/cleanup-newton' into 'master'

ref:pdelab/dune-pdelab> This MR fixes a couple of issues in the new Newton implementation:

• we add backwards compatible parameter names when working with a ParameterTree
• we fix a violation of the one-definition-rule in lineSearchStrategyFromString
• we add a backwards compatible handling of hackbuschReuskenAcceptBest

This MR fixes #165, #164, #163

These changes should make the transition for users significantly simpler.

See merge request pdelab/dune-pdelab!505

Closes #163 (closed), #164 (closed), and #165 (closed)

(cherry picked from commit b7d5dc31)

079b9371 [Newton] make LineSearchStrategy a class-enum to avoid namespace polution ceea91bd [Newton] put lineSearchStrategyFromString into an empty namespace 61e44f22 [Newton] fix function name in error message 679067f3 [Newton] add support for deprecated 'hackbuschReuskenAcceptBest' f1eadb56 [Newton,doc] fix documentation. The reduction is low by default! 97bb537b {Newton] use backwards compatible parameter names 57a14cca [recepie] update operator-splitting example to changes in NewtonMethod 5c80b8e8 Adjust test to backward compatible parameter names cd2bf3d7 Adjust doxygen documentation to backward compatible parameter names 35a41e04 Use consistent naming scheme for use_max_norm fef2d2f8 [newton] create _terminate before setting parameters a4b6fa0d Make lineasearch and terminate independent of Newton d6188da8 Backward compatible way of setting parameters through subtree