MPI resource handling

[lkotipal]

• finalize checks if communicators and datatypes are null before freeing, and sets them accordingly to prevent double frees.
• Destructor calls finalize. finalize can still be called manually in case object goes out of scope after MPI_finalize
• Implement copy constructor correctly duplicating MPI communicator and datatypes.
• Implement swap, and via that move constructor and assignment.
• Delete copy assignment since it's horrible memory-wise (particularly with copy-swap) and its use is likely a mistake anyway.

[ursg]

Looking good. Definitely nice to get rid of those warnings (and to feel a bit better about not potentially wasting performance there).

[ursg]

I know I approved this already, but double-approval looks even better!

[markusbattarbee]

🤔

[iljah]

since it's horribly slow

How slow is it?..

[lkotipal]

@iljah good point actually. I have absolutely not tested it, and it's more of an issue of memory footprint anyway for the canonical method. That would be constructing a temporary copy of the grid copied from and swapping with the grid copied to, meaning we're doing a whole bunch of allocation and holding an extra grid in memory.

[kstppd]

Although my opinion was not asked I agree with Leo. Although letting the option be there from the viewpoint of correctness makes sense this library is used only by Vlasiator and Vlasiator has special needs so this acts like a deterrent to the future coder.