Add functions to Myrial grammer
Rename unary operator to unary function in the myrial grammer
Build a function table for myrial functions
Add UDF invocations to myrial grammer
Test of invalid function name
Test of duplicately defined UDF
Test of UDF invocation with a missing argument
Add method to walk an expression tree
Implement add_offset in terms of walk
Re-write unboxing in terms of Expression.walk
Detect undefine variables in myrial functions
Test of a function with an undefined variables
Function to expand UDF function arguments
Test of UDF function composition
Test of nested function invocations
Remove some unused variables from datalog model
Use a function for distance calculation in kmeans
Better comment for add_function
Comment for resolve_udf
Check for duplciately defined variables