Rename: use dim_data_per_rank consistently

The most explicit and general way to specify a LocalArray's distribution is to use the `dim_data` structure from the Distributed Array protocol.  When we want to generate several LocalArrays from the client (usually just before packaging them up as a DistArray), we often need a sequence of these `dim_data` structures: one per MPI rank.  This structure is referred to as various things in our codebase:

* `dim_data`
* `dim_datas`
* `dim_data_seq`
* `local_dim_datas`
* `ddpp` (an abbreviation for an older term, `dim_data_per_process`)
* `dim_data_per_rank`

In this PR, I rename almost all of these to `dim_data_per_rank`, except for a couple of situations where I use the abbreviation `ddpr`.  `dim_data_per_rank` is not the most concise, but if we agree on a better term later, it will be easier to do a rename now that it's used consistently.