adding support for user-defined addons

The addons must be placed in the addons/ directory.
adding helper script to create an addon
remove unused PHONY targets
add missing PHONY targets
Bugfix: scale guesses might also contain LHInput symbols
allow for LHInput symbols in the initial guesses as well
test LHInput parameters in low-scale initial guess
fix memory leak

by not defining a variable of type std::string in a function which
might call exit();
adding get_high_scale() method to low-scale SG for compatibility
Merge branch 'development' into feature-fixed-point-iteration