Fixes YA compile issue of a sloppy use of "const" ... should compile

Fixes YA compile issue of a sloppy use of "const" ... should compile
now on all systems (with warnings possibly, but no errors). Mods which
force template expansion for a specific metric PDF function type
<float>, while still allowing for use-defined M-ary precision of the metrics.

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@3282 221aa14e-8319-0410-a670-987f0aec2ac5