Allow PooledDataVector construction using a given pool.

[doobwa]

In R it's useful to be able to provide a predetermined pool, e.g. x <- factor(1, 1:10) where the levels are 1:10.

This is a modified constructor to allow for this, but please let me know if there is a better way.

The other PooledDataVector constructor could use this one, but would amount to an additional pass through x.

[tshort]

I didn't try running it, but it looks good to me, Chris. It will be useful to have this.

As to the other PooledDataVec constructor calling this one, that sort of thing is generally okay. I think Julia is pretty efficient with function calls. For this particular case, the constructors are just enough different that it may not be worth the effort.

[doobwa]

Sounds good.