Add square dense matrix constructor reusing existing one.

This simplifies matrix construction in many places because in many cases we need just a bilinear form.
Add GlobalDenseMatrix::addSubMatrix for same columns as row positions.
Replace all addSubVector(), addValue() with add().

This helps with generalization when using essentially same concept of addition.
Add MathLib::RowColumnIndices representing rows and cols matrix indices.

This a lightweight pack-unpack class holding two vectors.
New add() function for global matrices using RowColumnIndices.
Add CRSMatrix::setZero().
Merge pull request #177 from endJunction/ExtendLinAlg

Extend MathLib linear algebra objects.