[Release] Merge branch 'feature/new-bcrsmatrix-buildmode'

This branch adds support for a new, implicit build mode to
BCRSMatrix and moreover improves error handling and the
build stage system used during matrix construction.

* feature/new-bcrsmatrix-buildmode: (35 commits)
  [BCRSMatrix] Fix a documentation typo
  [BCRSMatrix] Some small fixes to allocation and deallocation in implicit mode
  [BCRSMatrix] Prevent against accidental double allocation of row array
  [BCRSMatrix] Clean up deallocate() a bit
  [BCRSMatrix] New observer buildMode() to get at the currently active build mode
  [BCRSMatrix] Adjust existing build stage checks to new enum values and semantics
  [BCRSMatrix] Prevent changing the implicit build mode parameters after matrix construction has started
  [BCRSMatrix] Improve logic in setBuildMode() to correctly handle new implicit mode
  [BCRSMatrix] Prohibit copying into / out of partially built matrices
  [BCRSMatrix] Add checks against calling arithmetic operations on partially built matrices
  [BCRSMatrix] Make sure to always initialize all data members in constructors
  [BCRSMatrix] Introduce new build stages notAllocated (aliased to notbuilt) and building
  [BCRSMatrix] Export the type of the CompressionStatistics object
  [BCRSMatrix] Improve error reporting in BCRSMatrix
  [BCRSMatrix] Add safeguards for implicit mode to operator[]
  [BCRSMatrix] Improve error handling for implicit build mode parameters
  [BCRSMatrix] Fix typo in setImplicitBuildModeParameters
  [BCRSMatrix] Update maximum number of row entries in compress() on the fly
  [BCRSMatrix] Avoid off-by one iterators in compress()
  [BCRSMatrix] Rename BuildModeWrapper to ImplicitMatrixBuilder and add an extended constructor
  ...