Add CSR accessor methods

[KyleVaughn]

Description:

This adds functionality to the SparseMatrix library to access data stored in the CSR matrix subclass. This will be needed to perform the core sparse matrix-vector multiplication requirement of the SparseMatrix library. For a given (i,j) index, the corresponding element stored in the CSR format will be returned. If the data is not there in the CSR format, zero will be returned. Furthermore, a method to return the type of sparse storage will be implemented.

Tasks:

• [ ] If there is not already a getFormat() in the SparseMatrix superclass, define that method. It should return the enumerated format type.
• [ ] Check that the row and column indices are within the bounds of the matrix
• [ ] Index the rowPtrs array at the row specified
• [ ] Iterate through the colIdx array (or whatever the writer of the class calls it - I can check with them) until the desired column index is reached. If it isn't there then return zero.

Definition of done:

Must include getFormat() and operator()(size_t i, size_t j). The methods should be tested with the corresponding unit tests. Once tested, the implementation will be completed.

[KyleVaughn]

Review by @MeetMPatel

[MeetMPatel]

Clear and detailed explanation of the issue, tasks to be completed. Looks great!