Explain multi-patch domain decomposition

[yguclu]

Explain class MultipatchDomainDecomposition of module psydac.ddm.cart:

• Describe all attributes
• Describe how __init__ uses MPI groups & communicators to build single-patch DomainDecomposition objects

Explain function partition_procs_per_patch of module psydac.ddm.partition:

• Explain MPI partition algorithm
• Clarify that input and output parameters are global, i.e. the same for all MPI processes