many sumVars() calls

[mbouda]

sumVars called frequently leading to many strcmp checks of individual variables against variable list.
This become extremenly inefficient for systems with many variables in each layer equation (many root segments) prior to reduction.

May need to
--replace strcmp with more efficient original version
-- implement more efficient comparison algorithm in sumVars
-- weed out sumVars calls by tracking what gets duplicated and avoid checks altogether...
-e.g. check only list of most recently added variables (like 4, rather than 400), but then check every time (which may be doing anyway).