Data entry types

[mattwthompson]

Closes #2

[mattwthompson]

It turns out the implementation in Evaluator requires each property to be fully set up in openff/evaluator/properties/ before the corresponding ThermoML data can be parsed

[mattwthompson]

This test is failing (and probably the cause of other failures):

FAILED dimsim/_tests/datasets/test_thermoml.py::test_load_property_types[single_dhvap.xml-expected2] - Exception: No properties parsed

Dropping into a debugger indicates it's deeper:

dimsim/_tests/datasets/test_thermoml.py::test_load_property_types[single_dhvap.xml-expected2] > /Users/mattthompson/software/dimsim/dimsim/datasets/thermoml/thermoml.py(2041)from_xml()
   2040             import ipdb; ipdb.set_trace()
-> 2041             properties = _PureOrMixtureData.from_xml_node(node, namespace, compounds)
   2042

ipdb> node
<Element '{http://www.iupac.org/namespaces/ThermoML}PureOrMixtureData' at 0x16a186de0>
ipdb> namespace
{'ThermoML': 'http://www.iupac.org/namespaces/ThermoML'}
ipdb> compounds
{1: Compound(smiles=Fc1ccccc1Cl, index=1)}
ipdb> _PureOrMixtureData.from_xml_node(node, namespace, compounds)
ipdb> _PureOrMixtureData.from_xml_node(node, namespace, compounds) is None
True

This is the code most blatantly copied from Evaluator, a.k.a. I understand it the least

Note that similar tests for other properties are passing