Move LinearSystemAssembler and its test to AssemblerLib.

Cleaning up the files and corresponding directories as well as cmake inputs.
Rename files in Tests/AssemblerLib.
Remove serialExecute() overload for iterators.
Replace delegating ctor in LocalToGlobalIndexMap.
Replace delegating ctor in GlobalDenseMatrix.
Add explicit specifier to LocalToGlobalIndexMap(rows) ctor.
Compose vector/matrix builder in GlobalSetup structure.

Also add a typical setup using GlobalDense matrix and SerialExecutor.
Add static specifier to all GlobalSetup member functions.
Uncrustify TestSerialLinearSolver. Replace tabs with spaces.
Rename LinearSystemAssembler to VectorMatrixAssembler.

This reflects that this class can be used otherwise as only for
linear equation system assembly.