BUG: centralMass in dump

[aborderies]

Describe the issue:

When restarting from a different grid, I realized that the values given by image->centralMass (when restarting from a DumpImage) were totally random and didn't really make sense. I therefore have the impression that the gravity.centralMass variable is not correctly saved in the dump files.
I've added the line

data->dump->RegisterVariable(&this->centralMass, "centralMass",1);

just after line 31 in gravity.cpp and it seems to solve the problem.

Error message:

No response

runtime information:

Idefix version : V2.1.01

[glesur]

That's correct, thanks @Anto6453 . Can you open a PR with this fix, or do you want me to do it?

[glesur]

fixed in #287