Fix gtoval used in OBDM sample_onebody()

[cychow2]

OBDM sample_onebody() now calls eval_gto() with gtoval according to the basis (spherical or Cartesian) specified in the mol object.

[lkwagner]

This is a very fragile fix. If we ever use different orbital types or change anything, this will break. Instead you need to have another parameter that tells it about the gtoval parameter.

[lkwagner]

Thanks--unfortunately the use of private variables and type== cannot be accepted as is. Please change to be less fragile!

[cychow2]

I've made gtoval an additional parameter of sample_onebody(). Since it's also used in tbdm, I added a private variable self._gtoval in the tbdm class like I did for the obdm class, and pass it to sample_onebody().