Extend test suite: cover array inputs + all methods

[sadielbartholomew]

The test suite is overdue some love, notably because most of the > 50 methods in the Units class are not directly tested:

$ git grep "def " units.py | wc -l
65
$ git grep "def " test/test_Units.py | wc -l
5

where one of the latter five, test_Units_BINARY_AND_UNARY_OPERATORS, covers perhaps OOM 10 Units methods to some extent, but the rest focus on specific methods so still there is a deficit of ~50 methods to explicitly cover. And another of those five is very minimal as I just added it to expose 4060945).

Furthermore, the handling of NumPy objects is not tested at all (numpy is not imported in the test module). We should ensure this is tested where appropriate, e.g. to check that arrays are processed as expected.