VectorGridFunctionSpaces are uninitialized by default

Problem

!555 (merged) (more exactly: 8a735826) introduced a breaking change to my code. Since then, VectorGridFunctionSpaces seem to be uninitialized by default. My program works fine with a PDELab version before the mentioned commit. After the commit, I get UninitializedGridFunctionSpaceErrors before I can compute anything.

terminate called after throwing an instance of 'Dune::PDELab::UninitializedGridFunctionSpaceError'
  what():  UninitializedGridFunctionSpaceError [size:/home/lriedel/dune/dune-pdelab/dune/pdelab/gridfunctionspace/gridfunctionspacebase.hh:210]: space is not initialized

I did not find any documentation on this in the CHANGELOG.md or in the Doxygen documentation.

How do I now initialize the grid function space?

Proposals

• Document a breaking (?) change in the initialization of GridFunctionSpaces in the CHANGELOG.md.
• Add section in the Doxygen documentation on how to initialize GridFunctionSpaces now.
• Improve the error message and tell users how to circumvent the issue.