Implement some `tec.TECBase` attribs as @property
Implement some `tec.TECBase` attribs as @property

I implemented the electrode attributes, specifically `emitter` and
`collector` using the @property decorator. In this way I was able to
ensure that they can only be set with objects in the `tec.electrode`
namespace.

Closes #76.

jrsmith3/tec,jrsmith3/tec