Support decompositions with extra outermost dimensions in PIO2

[dqwu]

In PIO1, we can define an IO decomposition with extra
outermost dimensions (their lengths are all set to 1) to
write/read a variable with reduced dimensions. For
example, we can use a decomposition of [1 x 10 x 20]
for a variable of [10 x 20], or use a decomposition of
[1 x 1 x 10 x 20] for a variable of [1 x 10 x 20] (or [10 x 20]).

In PIO2, the above case is not correctly handled. With
buggy code like below, PIO2 can get mismatched
start/count values from the decomposition, and access
an array out of its bounds.

for (int i = 0; i < ndims; i++)
{
    start[i] = region->start[i];
    count[i] = region->count[i];
}

One E3SM developer test (ERP_Ln9.ne4_ne4.FC5AV1C-L)
fails (passes with PIO1) due to this confirmed issue.

We should add some FORTRAN unit tests to reproduce this
issue, and enhance PIO2 to support decompositions with
extra outermost dimensions.