Chunk get_data requests

[mullenkamp]

Currently, the get_data function sends one request to the Hilltop server for data. The request may be for 2 data points or 2 million. I find that some Hilltop servers struggle when the request is really big (e.g. > 1,000,000 data points). I also find that parsing the giant xml into pandas dataframes is sluggish and make the memory footprint of the dataframes larger than they should be.
Providing an option to chunk out a get_data request into multiple smaller requests based on a fixed number of years would solve these issues. I've already written some code for this in the past.