Add an example of a calculation with nprocesses>1

[valassi]

Just a reminder. In the epochX work #244, I kept several of the comments I had all over the code "FIXME this assumes nprocesses=1". I am not sure what the code should look like or how it should behave with nprocesses>1.

Eventually we need to add a test of one simple such example. This may even be a very simple process, as long as it has nprocesses>1.

@oliviermattelaer maybe this is for you to suggest one ;-)

Low priority anyway (Fortran integration is more important)

[valassi]

Maybe pp to tt? David suggested that it may be useful to try this anyway for other reasons.

[valassi]

Adding pptt is definitely needed - Olivier's bug #337 probably comes from this

[valassi]

In PR #396 for #343, I am removing mirror processes in uutt as done by @oliviermattelaer in branch 311. This means that now pptt subprocesses all have nprocesses==1.

Actually I have even added a static assert for nprocesses==1. To go beyond this, we should add an example with nprocesses>1.

Os, actually: was nprocesses>1 ONLY necessary for "mirror processes", and can this now be completely removed if these have been removed?... (question for @oliviermattelaer )

[valassi]

Is this also related to the difference between MAXFLOW and NCOLOR in the fortran? oliviermattelaer/mg5amc_test#21

[oliviermattelaer]

So in the latest gpucpp branch.
All P directories would have exactly one matrix-element function.
This matrix-element can still be related to multiple initial/final state but all of them will have the exact same matrix-element.

(Compare to my original email, this is now done in a "decent" way meaning that identical matrix-element are correctly handle)

[valassi]

Thanks Olivier! I updated to your latest upstream code in PR #619.

This is related to the discussion we had at the meeting earlier this wek https://indico.cern.ch/event/1263518/
You showed that for instance this process changes as follows

I could close this as fixed by PR #619. But I will keep it open for now, maybe I will clean up the code, remove a lot of nprocesses and/or add a few cleaner comments mentioning this #272 and PR #619