#1571 Make the way to reduce the objects to processor-wise and ghost much cleaner
#1545 (closed) has helped shape much more nicely what is really required to rebuild properly the models from prepartitioned data.
However, it underlines in the process how much brittle the code is in this reduction phase - I am struggling to keep track of everything and make sure nothing is lost in the process, but it is way too hard to do - and when I think I'm done it proves I couldn't be wronger (for instance a bug that appeared only with gcc 9 on FEdora highlighted new shortcomings.
The reason is mostly that up to my work on prepartition data I didn't need to bother much about the ghost elements on each processor - but it is no longer true, and adding them back as an afterthough is not easy. So I will provide for each main class (FEltSpace, GodOfDof, Mesh) a Reduce
method which will be in charge of ensuring everything is correct. This should prove easier to maintain in the long run.