Modify ResourceBuff::PopQty to be more exact.

Previously, resourcebuf would return to you a total quantity of resources +/- eps_rsrc of the quantity you tried to extract.  This is really annoying and so I have changed it.  It now does exact matching.  If you want to extract more than is there, you can't.  If you want to extract everything, then you can `buf.PopN(buf.count());.

Two examples of where the previous behavior is problematic:

* Impossible to extract a quantity less than eps_rsrc from a buffer.
* I am an enrichment facility.  I calculate the amount of natural uranium needed to satisfy an order of quantity X of arbitrary enrichment.  I extract quantity Y of necessary natural u from my natural u buffer, but my buffer quietly actually gave me slightly less than the quantity I told it to extract, Y'.  If the order enrichment happens to be for natural uranium, then X == Y.  If I every try to extract quantity X of the order's enriched composition from this popped natural u material, I get a negative quantity extraction error (rightfully), because that resource isn't actually qty>=X since Y' < Y.  So the enrichment facility would have to write special code to work around this behavior.