Add ELL accessor tests

[KyleVaughn]

Description:

This issue creates tests for ELL matrix access methods. The library is tested for being able to access a few elements related to the ELL matrix format - ELL column indices, and ELL matrix values. It also tests for the correct implementation for the state, and correct format, and prints relevant messages. Accessor testing is an important way to ensure the matrix information is stored and retrieved correctly.

Tasks:

• [ ] Collaborate with the people working on other ELL methods e.g.- ELL_accessor() method to determine the interface and calls.
• [ ] Define an ELL_accessor_test() method for testing
• [ ] Assert that n_rows,n_cols have been defined using the get() command
• [ ] Test the ELL column index matrix is of the right format: ell_colIdx(size_t i, size_t j) (needs to be decided by all of us working on ELL)
• [ ] Test the ELL value matrix is rightly defined: ell_val(i,j)
• [ ] Test implementation for state
• [ ] Test the correct format is returned for ELL
• [ ] Check all the outputs and print appropriate error messages

Definition of done:

The accessor testing method is complete when all the relevant checks for the accessor method, rows, columns, ELL column indices, ELL matrix coefficients, state, and format are completed and the relevant messages for these tests - "Test X passed/failed" (where X is the relevant operation defined in tasks) are displayed. This method is important for testing the functioning of all ELL methods.

[KyleVaughn]

Review by @Lazy-Beee

[Lazy-Beee]

For the description, I think you can focus more on what the test is besides why the test is important. This can give the readers a basic understanding of the task before they continue to the task list. The same for the 'definition of done'. It would be better if you could define what this test function should be able to do when it's completed.