Write compare_series() function

[andrew-edwards]

Aims:

• easily compare and plot values for two time series for the overlapping years
• ideally work across different types of data; could detect the class and then wrangle the data as necessary
• potentially expand to more than two series, so maybe do compare_two_series() first. Maybe have the wrangling as helper files.

Start with compare-npi-alpi.R code that I just wrote, based on Bridget's question.

[cgrandin]

See my comment in #50, you could do a similar thing to have a helper function for each type that puts them into a standardized format, so that you don't end up with one monstrous function that tries to do that with a ton of 'if' statements. Will save you a lot of debugging time down the road if you have small, simple functions of which likely only one will break at a time.