Make devicePersistent MPI aware

[greole]

Once #49 fixing #45 is implemented devicePersistentFields ie devicePersistentData<gko::Array<T>> need some way of handling parallel execution. This includes:

1. For local data, storing the MPI rank to which the data corresponds.
2. Additionally, some way to handle gathered/scattered global fields. If the sparsity pattern and matrix values are not stored directly as devicePersistentField and instead the CSR matrix is kept, only the initial values and rhs are devicePersistentFields.

Data	Local	Global	Location	Updated
lduCsrMapping	persistent	not needed	host	constant
initial guess	overwrite	persistent	device	constant
rhs	overwrite	overwrite	device	updated
matrix values	overwrite	overwrite	both	updated
globalIndex*	persistent	-	host	constant

Here, overwrite indicates that fields don't need to be stored, but storing would avoid reallocation. Global data is (for now) obtained by gathering the local data only. Thus, if global data is marked to be persistent (update false) a look up in the objectRegistry is performed.

[greole]

For now most of the functionalities related to distributed execution is handled on the ginkgo side via the partition and repartitioner class.