Add the verdi process call-root command

[sphuber]

Fixes #1150

This command will go up the call stack for a given process node and
return the root caller, if there is one.

[coveralls]

Coverage increased (+3.0%) to 73.76% when pulling cda258d on sphuber:fix_1150_verdi_process_call_root into 5e59da4 on aiidateam:develop.

[coveralls]

Coverage increased (+0.006%) to 73.957% when pulling 8401868 on sphuber:fix_1150_verdi_process_call_root into f83c67b on aiidateam:develop.

[giovannipizzi]

Wouldn't it be even more useful to implement instead a verdi callstack with the whole stack from root to current? (and maybe with an optional flag to see also called processes)? This would implicitly give the required info, but also much more

[sphuber]

The callstack is already implemented in verdi process status. This command serves simply to find the call-root. Its output is not graphical in anyway, it just prints a pk. The idea is that the result of this command can be piped into another one, e.g.:

verdi process status $(verdi process call-root 901)

[giovannipizzi]

I see. Thanks for the clarification!