Workaround for multicore ptasks.
Fixes https://framagit.org/simgrid/simgrid/-/issues/111.
Workaround for multicore ptasks (when user sets the same s4u::Host*
multiple times in the ptask).
The Element class in System.cpp links a variable and a constraint.
Ideally, we could have only 1 element for each pair variable/constraint.
However, the fair_bottleneck solver needs different elements to cope with multicore ptasks.
At FairBottleneck::solver, it considers each elem as a "variable" which must receive a full
share of the resource.
Multiple elements per contraint/variable isn't necessary for others solvers (maxmin, bmf).
However, it's probably not possible to use BMF at this moment
considering its instability and the current implementation in ptask_L07.cpp (especially the configuration of
task bound which is specific for FairBottleneck solver).
Partially revert change to set 1 element per constraint/variable pair (done at
ecb0903f).
