Parallel ISTL

This changes the linear algebra interface such that parallel linear algebra backends can be used and provides the implementation of the interface for parallel ISTL solvers.

Adds:

• Support for parallel ISTL solvers using the current solver interface
• Runtime switch for overlap type of the ISTL solver used
• solver->category: [default, sequential, nonoverlapping, overlapping] initfile parameter
• Comm object for the linear algebra interface that is to be used as a container for all required information for running the backend's solvers in parallel
• Implementation of the Comm object for ISTL

Changes:

• Linear algebra classes now use a Traits class as template parameter

TODO:

• Check if Grid uses overlap and use [Non]OverlappingSchwarzOperator
• Provide an efficient method for computing the parallel index set
• Resolve all TODO(FM) markers
• Make an alternative implementation for nonoverlapping Schwarz
• Handle the corner case overlapSize + ghostSize == 0
• Add an example or test file
• Update assembler element loops to use the proper partition set

Relates to #4 (closed).