Better memory management. Issue to be redistributed between more specific issues

[ppericard]

Find a way to improve memory management at nearly every step:

SortMeRNA runs are not memory capped, only the ref db index memory is capped, not the alignments memory

Ovgraphbuild could perform a treatment by bins instead of reading all the SAM file in RAM.

Unix Sort is sometimes used twice in a pipe command line so there is a risk of using twice the RAM that was given.