Add method to convert a sym to a pyobject

[cbm755]

This is for use with the Pytave project.

[cbm755]

This was #509, reopened after pytave merge

[mtmiller]

My only nits to pick are the names. The name pyobject isn't Matlab compatible, it's what we came up with to be the hidden implementation of PyObjects, which is fine, but it's kind of like an implementation detail at this point.

I also prefer have_pytave to be something else since the name pytave will be obsolete soon.

[cbm755]

I changed to have_py. I agree with the other point and have moved the discussion upstream: https://bitbucket.org/mtmiller/pytave/issues/61/have-subsasgn-pycall-others-cast-their

[mtmiller]

Thanks for the reminder about the upstream issue, I had been wondering what the use case would be, now I see that you want to be able to assign a sym variable into a container, and not that the user is calling the conversion method explicitly. So the intent is that this effectively acts as an implicit casting operator.