Add lis matvec solver - some improvements/changes/stuff to discuss

- Removed the update code for calculation of _max_diag_coeff from
  + LisMatrix::setValue()
  + LisMatrix::addValue().
- Removed the _max_diag_coeff attribute.
- Implemented the functionality in LisMatrix::getMaxDiagCoeff().

- Reformatted code
- Made docu more meaningful
- Changed name of first parameter of constructor
- Moved implementation of setValue() and getValue() from header into cpp-file

fixed issue in vtu-reader where declared precision of node coordinates was smaller than actual precision

Added check for zero elements in mesh read method

Improved speed, removed old code - very good!

Most points in discussion are a matter of programming style (using for_each or for loops, using or not using lambdas). In my opinion there is not a design error and for this reason I merge the pull request.

Fix triangulation bug