checking in some utilities which fetch census data from the cloud

also a notebook which demos how this works
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tested for whole county of data - works fine
adding unit tests for census helpers (100% coverage)
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add census to the list of dependencies
I think I needed census>=.6
make numexpr a dependency
census .5 is the latest?
categorizer is working for acs data
adding categorizer
applying categories to disaggregate data (pums records)
annotate the notebook
I think I figured out how to use census .5
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Merge pull request #5 from synthicity/census-cloud

helpers to get census data into data frames from the census rest api
Merge branch 'master' into categorizer
adding categorizer module - should be fully tested
add the tests
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