Test convergence of scale in Convergence_tester_skeleton

Since it checks convergence of DRbar parameters.
Rename Convergence_tester_skeleton to Convergence_tester_DRbar

Convergence_tester_DRbar is supposed to be a base class of convergence
testers that examine scale-dependent quantities such as DRbar masses.
Make Convergence_tester_DRbar available to other methods

The Convergence_tester_DRbar class template in
src/two_scale_convergence_tester_drbar.hpp is now a partial
specialization of that in src/convergence_tester_drbar.hpp.
Merge branch 'convergence-tester-drbar' into development