Generate a pool of conformers

Fix for #8. Generates a pool of conformers and selects from that pool. Pruning by RMSD is performed twice: once during the call to EmbedMultipleConfs to generate the initial pool, and again (optionally) after minimization of the conformers.

The expense associated with generating the initial pool can be modulated using the `pool_multiplier` argument, which defaults to 10x the number of requested conformers.