Fix some warnings in MATLAB code.

These are just a couple of things I ran into while working on Octave support.

* The first commit fixes a warning when calling `is_function_processor_working` -- it was trying to call `addpath` on a path that doesn't exist. The function still worked because everything under `matlab/` is on the path anyway.
* The second commit fixes some warnings on Octave; Octave gives a warning if the name of the function doesn't match the file name, whereas MATLAB just silently uses the file name.
* The third commit is a small cleanup -- it seems a little strange to be doing `feval(str2func('f'), ...)` instead of just directly calling `f(...)`.