Improve the documentation of ConsensusAlgorithm

[peterrum]

While revisiting the ConsensusAlgorithm documentation I have noticed how bad the documentation is. Since this feature is used in increasing number of places (which is the reason why it also caused a lot of headaches to @tjhei) and should be used in as many places as possible in large scale simulations, a good documentation is quite important. A PR with a new use case is in the pipeline!

Related to the documentation, the method name are also somewhat intuitive. I would like to rename some methods of the class ConsensusAlgorithmProcess :

• pack_recv_buffer -> create_request_to_process
• prepare_recv_buffer -> prepare_buffer_for_answer_from_process
• process_request -> answer_request_to_process
• unpack_recv_buffer -> read_answer_from_process

The old names probably had some reasons half a year ago but is kinda counter intuitive and I am forced to have a look at their meaning every time I implement a new instance.

I will also add some pictures related to PEX and NBX.

I will tackle these issues in multiple PRs (when I have some free time).

ping @kronbichler @bangerth