Feature idea: Calculate margins of error within the library

[chekos]

Got an email about calculating margins of error within the library.

Currently all pypums does is retrieve the ACS data from the census server.

I've wanted to extend the library to include some common calculations or data transformations (i.e. extract a county's data, grab only 25-55 year olds, etc.) Calculating margins of error is very common and including it on pypums could be really useful.