Clarity in documentation and code around chunksizes

1. If you search the document for chunksizes you get a lot of stuff about dask chunk sizes first. Can be confusing if you are interested in variable chunksizes.

2. The method v.data.set_nc_hdf5_chunksizes actually sets the chunk shape, not the volume/size.

E.g. the word size is clearly about the volume, as can be seen from the documentation:

https://docs.unidata.ucar.edu/nug/current/netcdf_perf_chunking.html says "Currently the netCDF default chunk size is 4MiB The current default chunking strategy of the netCDF library is to balance access time alongany of a variable's dimensions, by using chunk shapes similar to the shape of the entire variable but small enough that the resulting chunk size is less than or equal to the default chunk size."

Start by searching the documentation and code for “chunksizes” and inspect v.data.set_nc_hdf5_chunksizes. Compare the references to Dask chunk sizes with variable chunk shapes and sizes. Done means the terminology and documentation clearly distinguish these concepts and accurately describe what the method sets.