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 allTODO(FM)
markers 
Make an alternative implementation for nonoverlapping Schwarz 
Handle the corner caseoverlapSize + ghostSize == 0

Add an example or test file 
Update assembler element loops to use the proper partition set
Relates to #4 (closed).