item_gen produces b parameters outside b_bounds

[tmatta]

When specifying b_bounds, item_gen() can produce values of b that are outside the specified bounds.

[tmatta]

I reworked how the partial credit items are built. The category parameters are now built from the item-specific mean rather than the pool-specific mean. There is no more centering at all. The code from KJ no longer has issues. However, previous uses will not replicate as its a literal bug fix... No arguments were added to item_gen.