Release numbering

[luca-heltai]

We should follow the release numbering of deal.II, i.e.,

pi-BEM v9.0.0 -> works with deal.II 9.0.0 and deal2lkit 9.0.0

[nicola-giuliani]

That seems a good idea, i think that the master branch is ready to be released as 9.0 (even if it compiles and run with 9.1)