Better python printing

[gchenfc]

Since gtsam::KeyFormatter is a boost::function and pybind11 does not do boost::functions, some modification is needed in order to be able to use the print_(string s, KeyFormatter) function. Specifically, the print_ function is altered so that the python interface accepts a std::function and then calls the gtsam print with the boost version. So for example:

class ClassWithPrint {
  void print(const string &s,
             const gtsam::KeyFormatter &keyFormatter);
};

...
        .def("print_",
                    [](gtsam::ClassWithPrint* self, const string& s, const std::function<std::string(gtsam::Key)>& keyFormatter) {
                        py::scoped_ostream_redirect stream;
                        self->print(s, [&keyFormatter](gtsam::Key key){return keyFormatter(key);});
                    }, py::arg("s"), py::arg("keyFormatter"))

Note: py::scoped_ostream_redirect stream; is so that printing occurs to python sys.stdout which just generally plays nicer with python. See pybind docs

Also, this allows functionals for the keyformatter:

def myKeyFormatter(key):
    return 'this is my key formatter {}'.format(key)
factor.print_('factor: ', myKeyFormatter)

outputs:

factor: min torque factor
  keys = { this is my key formatter 23924273508777984 }
  noise model: unit (1) 

[varunagrawal]

Partial review. I am confused why not just add support for functionals?
https://pybind11.readthedocs.io/en/stable/advanced/cast/functional.html

Also, you've mixed up two PRs. Please separate, or wait till I add functional support, or even better, discuss?

[varunagrawal]

What's up with this? Not part of this PR.

[gchenfc]

my bad... temporarily i've just changed the base of this PR, but if this goes through before the other PR then I'll make a new branch/PR to merge

[varunagrawal]

Why make this gtsam::KeyFormatter and not a functional?

[gchenfc]

not sure I understand your question - gtsam::KeyFormatter is typedef'd to boost::function, so it is a functional? Unless that's not what a functional is...

[varunagrawal]

Not a good idea. If a class defines a print() method, this might override that (or the other way around).

[gchenfc]

Once borglab/gtsam#735 merges, this will no longer be necessary so I'm closing this PR.