WIP: Added benchmarks for Table

I've started adding some speed tests for the Table class (since there is some refactoring planned for 1.0). We should agree on the benchmarks before merging, so @taldcroft and @mhvk, do you think the current benchmarks here are useful? What other kind would you add for now?
And add a case for computing the mask `self.table['a'] > 0.6`.