Week 3 - GSoC '21

I have worked mainly on PR #21653 in Week-3. In This PR, I have managed to:

1. Add a new and improved TransferFunctionMatrix class. It is fast and uses 50% less code than previous implementations. Along with this, I have added comprehensive documentation with examples at the class level as well as for all the public methods/classmethods (from_Matrix(), var, num_inputs, num_outputs, shape, transpose(), elem_poles() and elem_zeros()).
2. Refactored the old test suite for TransferFunctionMatrix and made it more effective and fast at the same time. I removed redundant tests.

Apart from this PR, I have parallelly worked on Series and Parallel classes for MIMO systems. I will open the PR for the same as soon as the pending PR is merged. I will also open PR for graphical analyses with this.