Refactored is_x boolean methods in unit module.

This PR contains fairly minor code changes pertinent to this Iris [forum topic](https://groups.google.com/forum/#!topic/scitools-iris/LLSKr47kg2c).

To achieve consistency, all of the `is_x()` functions are now methods. Previously, some of the functions were implemented as properties using the `@property` decorator.

All of the unit tests and doctests passed with these code changes.

I couldn't find any references to the affected methods in the sphinx documentation (e.g. the user guide), so no doc changes have been made.

I have used the `deprecated:: 1.3` stanza in the relevant docstrings (of the old methods). However, it occurs to me that this PR may go into a 1.2.x release. In which case these stanzas will need to be fixed.

As this is my first time around the (devilishly complicated) Iris dev loop, it is almost certain that I have missed something :-)

Big thanks to bjlittle for assistance and patience.