Removed "^:const" attribute from 3 variables.

"^:const" allows inlining, but these need to be redefinable during
initialization without replacing constants.clj.  Also removed plus signs
from the ends of the names.  The three vars were:

+dir+
+analogy-to-propn-pos-multiplier+
+analogy-to-propn-neg-multiplier+

It's usually OK to redefine vars during initialization, but usually not
in the middle of a run.  See
http://stackoverflow.com/questions/5181367/is-defn-thread-safe .