multiRead is possibly wrong

[ykempf]

In the current form, the multiRead with offsets I had tried to deploy along with Vlasiator still only does one multiReadUnit at a time, and that works though defeating the point of trying to mash the units into one read.

https://github.com/fmihpc/vlsv/blob/master/vlsv_reader_parallel.cpp#L380-L394

Here I think there is a mixup of pointers to the ->array members of the multi read units which point to the buffer allocated in user code to get the data read, and pointer offsets that should map to the offsets we compute at great length in the user code. I don't think that the datatype we make with MPI_Type_create_struct should be getting pointers to the destination buffers. multiReadOffsetPointer should be passed for the destination buffers at https://github.com/fmihpc/vlsv/blob/master/vlsv_reader_parallel.cpp#L399, that I agree with.

Posting this for posterity, we won't address this right away in relation to Vlasiator.