Investigate how expensive sqrt is for CUDA (registers) and C++ (vectorization etc)

[roiser]

(more generally, check all arithmetic operators)

Looked into by Andreas (either pass 3 momenta and calculate the 4th or pass 4 momenta right away)

TODO (understand what’s better 3 or 4 momenta)

SR looking at all +,- etc operators, they do take up registers...

[valassi]

Changed the title as investigating alternatives to sqrt is relevant not only to CUDA but also to C++. See the discussion on vectorization optimizations in #173 and the nice advice by Sebastien.