implement collector accumulator as in #349

[wiso]

This is a simple implementation of collector accumulator, a very simple accumulator which stores all the values passed, as suggested in #349. Some new tests are implemented. Documentation is updated.

I would like to have a code review. For sure these items are missing.

• [ ] update Changelog
• [ ] add copyright
• [x] check compilation with b2

Probably implementing serialization for this is not a good idea.

I had to change the continuous integration:

• fix how libraries are searched by depinst.py, since by default the example directory is not used
• move to Ubuntu 20.04 since 18.04 is no longer supported https://github.com/actions/runner-images/issues/6002. Because of that also moved from gcc5 to gcc7.