Prefix functions in validator class with is_

[eonu]

As a big Ruby fan, I miss the way you can tell a function returns a boolean if it ends with a question mark (e.g. 1.even?).

With the _Validator class, instead of having:

self._val = _Validator()
self._val.restricted_float(...)
self._val.func(...)
self._val.iterable(...)
self._val.fitted(...)

It would be clearer if we prefixed these functions with is_ to make it clear that they are performing some check (though not necessarily returning a boolean like the Ruby ?):

self._val = _Validator()
self._val.is_restricted_float(...)
self._val.is_func(...)
self._val.is_iterable(...)
self._val.is_fitted(...)

Also a more appropriate name for func would be callable.

[eonu]

Implemented in #161.