Feature/observer

This MR replaces the current mechanism for automatic updates of Dune-bases and DOFVectors with an observer pattern mixin.

Adds:

• AdaptiveGrid wrapper class to be handed out to the user. This allows addition of functionality to Dune-grid types. It is expected to be used instead of a regular grid by the user whenever possible.
• ParallelGlobalBasis class expected to be used instead of a regular Dune-basis. The Communication object is now paired directly with a basis within this object.
• Mixin classes Observer and Signals for adding automatic update functionality to other classes.
• Observer unit test
• SharedPtr.hpp providing wrapping of objects and shared_ptr/unique_ptr into shared_ptrs
• free function makeGlobalBasis to generate a ParallelGlobalBasis from a prebasis

Removes:

• GridTransfer[Manager].hpp since those are no longer used.
• DOFVectorInterface.hpp

Changes:

• Lots of changes due to feature/petsc_backend, see !78 (merged).
• Interface changes to several classes to use the new Observed[Grid, Basis] objects when needed.
• DOFVector now creates a new DataTransfer on copy. This change is independent of this MR and may be cherry-picked.
• Restructured preAdapt/adapt/postAdapt callbacks in DataTransfer and DOFVector
• MeshCreator now returns an AdaptiveGrid

TODOs:

• Cleanup of initial commit
• Clearer names for the new classes
• Test of Observer hierarchy (skipped subjects, multiple subjects) - see discussion below