Python UDF and C++ UDF improvements

[mxwli]

Changes:

• UDFs now evaluate to null by default when any one of the parameters is null. This is changeable via the default_null_handling parameter
• Exceptions can now be caught from python functions via the catch_exceptions parameter. When an exception is caught, the function is evaluated to null
• UDFs can now be removed, both in the C++ and in the python API via removeUDFFunction and remove_function respectively.

C++ UDF API now supports zero parameter functions, but this is an experimental feature (ie. not tested very well)