Updated how to holf checkpreemptfn in ikfast, eliminating weakref

[cielavenir]

In this way we can remove weakref.proxy(self).func (and we don't have to put weakrefmethod as an openrave dependency)

Already reviewed by @woodychow .

[rdiankov]

please don't have checkpreemptfn initialized in the parameter list like this, it looks weird and bad..

[rdiankov]

ping