[bugfix][parallel] Fix partition in rangeSegment() function

This function should distribute a range of length size into n segments of approximately the same size. The current version uses segments of length floor(size/n). The remaining r = size % n entries are added to the last segment.

For small segment sizes and large modulus r = size % n this may lead to extremely bad partitions. E.g. for size=17 and n=9 we get floor(size/9) = 1 such that we have 8 segements of size 1 and one of size 9. In contrast, if we use n=8 we get 8 segments of size 2. I.e when going from n=8 to n=9 threads, the runtime will be about 4 times as long.

This patch changes the partition to use r segments of size ceil(size/n) and n-r segments of size floor(size/n) which is as balanced as possible.