Split RPL into core and mode related functions.
Merge pull request #1167 from fabianbrandt/rpl_base_split

Split RPL into core and mode related functions.
sys/ps: s/float/int for runtime

The high precision isn't needed, use per mille instead.
Circumvents printf problems on some platforms.

Also: prevent division by zero.
sys/ps: fix comment style, superfluous declaration