Binary op on two datarrays with different ticks does not raise an error

[kwgoodman]

No error is raised for binary operations where axis labels match but ticks don’t:

>> dar1 = DataArray([1, 2], [('time', ['A1', 'B1'])])
>> dar2 = DataArray([1, 2], [('time', ['A2', 'B2'])])
>> dar12 = dar1 + dar2

An error is raised for binary operations where axis labels don’t match but ticks do:

>> dar1 = DataArray([1, 2], [('time1', ['A', 'B'])])
>> dar2 = DataArray([1, 2], [('time2', ['A', 'B'])])
>> dar12 = dar1 + dar2
<snip>
NamedAxisError: Axis labels are incompatible for a binary operation: ('time1',), ('time2',)

[kwgoodman]

Closed by 892517c. Binary ops on mismatched ticks now raises.