makePrimesTree in fig_2_18 and fig_2_19 fails to populate tree with all values

[jnorwood]

makePrimesTreeElem needs to have IntVector::iterator &i as last parameter and makePrimesTree needs to pass in a reference to copy of vec.begin() that can be modified.
`IntVector::iterator it = vec.begin();

currently the iterator parameter is constant, and it fills the tree with duplicates of some data.

I'll attach a copy of fig_2_19 that has this fixed. The same fix needs to go into fig_2_18.

fig_2_19b.txt

`