when opening a subfile, fp->nldset and fp->nmdset are not read from
subfiles. Instead, they are read from the master file. This commit
fix this issue and read from the subfile correctly.

It is possible that, for example, a file is created with 8 subfiles.
But when openning and reading the file, we only use 4 processes.

In the original implementation before this fix, the info of 8 subfiles
is not saved. Only the first fp->ngroup subfiles will be opened for
read, where fp->ngroup is a number bounded by the number of processes
(i.e. <= 4 in this case).

In this fix, we use fp->nsubfiles to store the number of subfiles for an
opened file. All fp->nsubfiles subfiles will be opened for read.

MPI_FILE_set_view is a collective call. Befroe this fix, not all
processes call this function during dataset read, introducing possible
hangs. A subfile read test case may trigger this issue more easiliy.
This commit fix this issue.

Test the following:
1. nsbufile > nproc
2. nsubfile == nproc
3. nsubfile < nproc

For each of the above, test:
1. read pattern same as write pattern (row wise)
2. read pattern is row wise, but each process read a different row than
it writes. (read from one subfile that process is not responsible for)
3. read pattern is column wise (read from several subfiles)
4. read all dataset.

This means a total of 12 scenarios are tested.

Also, we test each scenario using 1 to 12 number of processes. This
makes sures Log VOL also works for odd number of processes.