checks should have an apply method

[Zaharid]

... Especially argchecks.

When the make_argcheck decorator is applied, the result is a function that takes another function as argument, and instructs reportengine to apply the corresponding check. However we may be interested in simply using the function outside the loop. A possible solution is to make it so make_argcheck adds an apply method to the checks, that has the same effect as the original function.