Modified mean function

Added a new mean function having a scalar output.
Added isScalarFunCall

This method checks whether there exists a specialised method for scalars and return a boolean value depending on the result.
Added isScalarLibCall

Added the method declaration to the header file. Changed name of isScalarLibcall to isScalarLibCall.
Removed handleScalarLibCall

Removed the method since it was not being used.
Modified funCallExprCodeGen

Added a new parameter for the lhsExpr the default value of which is set to NULL.
Modified funCallCodeGen

Added  a missing ampersand to the lhsExpr since a reference to the variable has to be passed.
Modified assignStmtCodeGen

Added a check for scalar function calls.