Make the sym class a new-style classdef class

[cbm755]

Possibly this allows fixing #13.

Good chance to learn more about Octave OO programming. Maybe @latot is interested?

[latot]

Yay! i'm interested, and to start: https://es.mathworks.com/help/matlab/matlab_oop/getting-familiar-with-classes.html

We must be careful, i don't know if all functions of classdef are available in octave.

[cbm755]

You might look at @pyobject which is classdef and works properly:

https://bitbucket.org/mtmiller/pytave/src/d0a7f66393fcaeaad789d83c41462ed6a4c353ed/@pyobject/?at=default

[mtmiller]

There are some limitations in the Octave implementation of classdef to watch out for. The simplest thing to do would be to change @sym/sym.m to define the class and constructor only, and all methods defined in separate .m files should still work as-is.