Warn on a bad signature for an 'observe'-decorated function

[mdickinson]

This PR adds a runtime warning for an observe-decorated method whose signature is incompatible with 'observe'.

I'm in two minds about whether this is a good idea - it's unnecessary runtime overhead, but on the other hand that runtime overhead should be small and will typically only apply at import time.

One the other hand, forgetting the event parameter is a very common mistake with observe-decorated methods, especially since the event-less signature is compatible with on_trait_change.

I haven't extended the fix to dynamic applications of observe: a.observe(handler, expression); that does seem like a case where we don't want the extra runtime overhead, since there may be many hidden calls to observe in a codebase, but occurrences of the observe decorator are limited in comparison.

Closes #1428.

Checklist

• Tests

[mdickinson]

This caught a bug in Traits' own test suite, here:

traits/traits/tests/test_has_traits.py

Lines 190 to 191 in c791f76

	def handler(event):
	pass

Given that, I'm inclined to keep the warning.

[mdickinson]

This caught a bug in Traits' own test suite

Fixed in #1530.

[rahulporuri]

LGTM with one question/request for clarification