Add DEN accessor methods

[KyleVaughn]

Description:

The library needs to be able to get information about matrices in the dense format. This matrix information will be important for performing dense format operations.

Tasks:

• [ ] Be able to determine the number of rows for a dense format matrix
• [ ] Be able to determine the number of columns for a dense format matrix
• [ ] Be able to determine the element value at given row and column index values for a matrix in the dense format
• [ ] Be able to set the element value at given row and column index values for a matrix in the dense format
• [ ] Be able to return a "DEN" label for matrices in the dense format

Definition of done:

Evidence of a functional accessor implementation that is able to return index, size, coefficient, and format information about matrices in the dense format

[KyleVaughn]

Review by @rgrawe

[rgrawe]

This looks good. Includes all the necessary tasks and each task is well defined.