refine rRNA detection

[oschwengers]

A refinement of the rRNA detection is necessary as currently too many truncated and false positive sequences are predicted.

Inspired by Barrnap and certain issues (tseemann/prokka#423, tseemann/barrnap#39), the cmsearch based approach (cmsearch --noali --cut_tc --notrunc) therefore gets replaced by a cmscan approach in glocal mode (cmscan --noali --cut_tc -g --nohmmonly --rfam)