Make loading more robust

[ranocha]

As discussed with @JoshuaLampert and @sloede, it would be nice if we could make the loading infrastructure more user-friendly. Right now, the code

P4est.jl/src/LibP4est.jl

Lines 15 to 25 in 8202433

	@static if _PREFERENCE_LIBP4EST == "P4est_jll"
	const libp4est = P4est_jll.libp4est
	else
	const libp4est = _PREFERENCE_LIBP4EST
	end
	@static if _PREFERENCE_LIBSC == "P4est_jll"
	const libsc = P4est_jll.libsc
	else
	const libsc = _PREFERENCE_LIBSC
	end

will fail if we cannot load P4est_jll.libp4est while P4est.jl is configured to use the JLL (default). Thus, we must first set the P4est.jl preferences and afterwards the MPI preferences (see #90). Moreover, one must always set P4est.jl preferences when custom MPI preferences like these are set if P4est.jl is loaded - even if P4est.jl is not used at all.

It would be much nicer if we would just print a warning if we cannot load libp4est but allow using P4est. Ideally, this should also be done similarly with T8code.jl.