Areas not covered by code

[hyanwong]

There are a number of cases that the test suite doesn't cover. This issue lists a few, that should be added to unit tests:

• test a case where a binary node turns into a unary node in a later tree
• test a case where a sample node is internal (not sure if this is allowed, though)
• test for sensible failure when a node_to_date has no children (i.e. a dangling node)
• test for success when a node_to_date has a single, dated (sample) child (i.e. a unary node above a sample)
• the functions forwards_mean_var & restrict_ages_topo seem not to be covered in test_functions.py. They should be, and the nodes_to_date parameter tested
• the caching of node mixtures is not tested, as none of the tests have duplicate node mixtures (this should be easy to test). Also there are no tests with large mixtures (number of descendant samples > 5)