Add a COO matrix class

[KyleVaughn]

Description:

Add a COO matrix class, which is a child class of the SparseMatrix class.

Tasks:

1. Create COO matrix format interface --- SparseMatrix_COO.hpp

   private: size_t I, size_t J, fp_type* val

   contain implementation for assembleStorage()

   add getter method for I, J, and val.

   add default constructor

   add parametrized constructor

   add destructor

2. Create COO class constructor/destructor --- SparseMatrix_COO.cpp

   implement default constructor

   implement parametrized constructor

   implement destructor

Definition of done:

1. Must include a default constructor and a constructor which takes one argument per member variable and directly assembles the class.
2. getter methods must be implemented for private members
3. Whole implementation should pass tests.

[amirnzm]

I am reviewing this.

[amirnzm]

The issue description, tasks, and definition of done make sense to me and is good to go.