Clean up use of generics for the curves/pairing groups

[AntoineRondelet]

For now there is still quite a lot of code duplication that do not allow a clean management and instantiation of the pairing groups we want to use.
That would greatly improve the readability/flexibility of the project to write more generic code w.r.t the pairing groups used and facilitate its instantiation with concrete groups. That likely requires to add extra layers of abstractions (which will yield more overhead - this needs to be measured).