changed num_density_to_mesh to take the readlines() from alara output so all processing can be done in memory with system calls rather than writing output to a file then rereading it.
added duck typing support, now files or lists
resorted to popping with most of the previous structure intact. ready for a review, @scopatz.
Merge branch 'alara_inmem' of https://github.com/gidden/pyne into gidden-alara_inmem