Add Cypher Distance function and tests

Distance function calculates Euclidean distance between two points.
Distance(a,b) where a and b are expressions that evaluate to either
Node or String. Node contains x,y attributes. String is of WKT
POINT(x y) form.
NOTE: Test for this version fails where a,b are Nodes, possibly
not properly mocking the Node correctly in DistanceTest
Add Distance to known functions for Cypher compiler