Add reverse lookup of rates

[alecloudenback]

Given a rate, list the table and ages for which the rate occurs.

[alecloudenback]

Would want to return all of the following in a parseable format:

• table name
• table number if available
• select/ultimate
• age
• duration

Also, the function would only be as good as the set of tables loaded. With the relatively small subset of bundled tables, has limited utility.

[alecloudenback]

This now would have a lot more utility since all tables are bundled and parse-able. The return data could be an array of named tuples.

[alecloudenback]

Also, add a keyword argument for tolerance (abs and rel) - maybe just perform the comparison with isapprox