Allow XPath references to model objects, not just attributes

[luciansmith]

SED-ML explicitly allows setting and getting the mathematical value of model elements, irrespective of their attributes. A variety of Biomodels SED-ML files rely on this, and the number will only increase going forward, as parameter scans and other similar tasks become more common. The first step is simply to remove the various:

            if sep != '/@':
                raise ValueError('target {} is not a valid XPath to an attribute of a model element'.format(change.target))

errors from sedml\utils.py, but more downstream work is almost certainly needed, as well, and may extend to the various biosimulators_[simulator] projects.