> Use of MethodType, and adding methods to instances, seems questionable. Is this the only way?
I couldn't find any official documentation for the MethodType constructor,
so how are people supposed to be certain of what the args are for?

```Python
>>> import types
>>> help(types.MethodType)
Help on class instancemethod in module __builtin__:

class instancemethod(object)
 |  instancemethod(function, instance, class)
 |  
 |  Create an instance method object.
...
```

> Did you consider wrapping or subclassing netcdf.datetime instead?

Subclassing was done for the `datetime` object since it's immutable.  Since I wrote this in a general manner, any object which can hold time information can then be translated to a numeric value and holds unit information.  Is there an advantage to subclassing here?  I considered the overhead of creating a new object with additional functionality not to be necessary, especially as everything within `_datetime_numeric_comparison` only exists as a temporary way of handling current iris behaviour i.e. this modifying of objects and subclassing will not be required in the case when we remove the ability to constrain based on a numeric values for times.

As a side to this, I'm considering moving `_datetime_numeric_comparison` to the `pdatetime` module.