Remove duplicated code and consolidate coo2csr functionality in a single function.

[pelesh]

Consolidate coo2csr functions in MatrixHandler and Csr classes into a single standalone function in matrix namespace. This utility function is now called by both, MatrixHandler and Csr classes.

This is first step before refactoring coo2csr function, as well as matrix::Sparse class to address #166.

[superwhiskers]

without suggesting any additional modifications to the code, i ask that you move the utilities.cpp & utilities.hpp files to Utilities.cpp and Utilities.hpp, as in #164.