Added sos2tf() function to convert SOS to Transfer Function coefficients like matlab's SOS2TF

[yangbo]

Hello, I have added some features in my fork of this project.

They are:

1. Added sos2tf() function to convert SOS to Transfer Function coefficients like matlab's SOS2TF.
2. Added conv() function to do convolution.
3. Added CMake as building system, and move tests to 'tests/' subdirectory. Because I do not use SConstruct so I didn't change it, but changed the doc to cmake.
4. Added some cross-compile support for arm64 target, but they are specific to my development environment.

If there are something wrong, please tell me. : )

[ruohoruotsi]

My sincere apologies for taking a long time to update the project and consider your thoughtful PR. Thank you for your contribution.

I also just noticed the CMakeLists.txt changes in this PR. Because my changes of this evening introduce merge conflicts, I'll address these in new PRs, where I would like to accept your conv and sos2tf changes along with my current updates to support a better, generalized build system, with more tests (validating incoming fixes) as well as adoption of Github actions and generally an up-to-date, modern C++ repo

[yangbo]

hi @ruohoruotsi, it's ok and thanks for your work, it provides a great reference. I have used it to make my work done. Thank you!