Convenienience: access to all base blocks and partition by number of packs

[pgrete]

The "base" MeshData container may not be populated by default and we have a couple of places in the code where we need all "base" blocks in a single container (currently safeguarded by GetOrAdd("base",0) with a check for num_partitions=1 (i.e., pack_size=1).
The nicer option would be to introduce a GetOrAddAll that puts all blocks in a single container.

Separately, it would be nice to specify the number of packs rather than the pack size as this would allow for more flexible partitioning in particular for simulations with varying number of blocks.