Change some doubled on statements into single on statements

Sung suggested combining these on statements, a good idea.
Change an expression that could involve communication into a known local one

Sung suggested this simplification to a conditional expression.
given: const hi = this.domain.hi - count;
       if pos+count > this.domain.hi then ... -> if pos > hi then ...
Serailize the array count method.

I originally implemented it using a parallel construct, but Sung pointed out
that it was the only parallel function in this set of list/vector functions
on arrays. Rewrite it as a for loop.
Rename one of the classes in the test timeVectorArray

Renamed the class SumReduce to SumElements.
Add a comment before list/vector methods on arrays

Add a comment about the list/vector methods describing what they are and what
limitations they have (not parallel safe, reallocate with every domain
modification).
Remove assertSingleArrayDomain in favor of chpl__assertSingleArrayDomain

Ben used assertSingleArrayDomain out of my branch in his set operations code,
then I renamed it with the leading "chpl__".  For a little while both existed
on my branch.