Remove Function::operator[+-*/]

[Jan Blechta]

Operators Function::operator[+-*/] return FunctionAXPY object holding (no delete shared) pointer to *this. Once *this is destroyed FunctionAXPY object is invalid and will segfault on its use. This is unfixable remnant of #688. this is only plain ptr in C++.

These operators might not be exposed on the Python side so there would be no harm to the Python interface. Otherwise they might be easily reimplemented in Python or SWIG layer by passing shared ptr version of this to FunctionAXPY constructor.

[Jan Blechta]

49b7e1ad160e1a35e129938e702fa480e228677e, a7ed5710e2a3a4197a2930a59f872c8b36446a03