[boost] uBlas and Google Summer of Code 2020

Hi Boost community, I've added new projects for the Google Summer of Code 2020 for the Boost uBlas library (matrix computations, tensors and linear algebra). If you're a student and wish to candidate, please contact me as soon as possible. The list of projects is here: https://github.com/boostorg/wiki/wiki/Google-Summer-of-Code%3A-2020 Cheers, David