Add function to count data sets

[tomschenkjr]

Counting the number of data sets on a Socrata data portal is more nuanced that it seems at first. Socrata portals include filtered views and other subsets that aren't really distinct "data sets".

@levyj created a method of counting the number of data sets that we use in the City of Chicago:

1. Use https://data.cityofchicago.org/resource/7eck-a4hy.json?$select=type,count(type)&$group=type to download the catalog, aggregated to view type.
2. Sum the Tabular and Blob counts.
3. Determine the number of Mondara datasets with https://data.cityofchicago.org/browse?tags=map_layer​ and add that count to the Tabular + Blob total.
4. This final total amounts to Tablular datasets + Maps in KML/Shapefile format + Maps in Mondara format.

It would be great to implement a function which includes all of these steps. It will help ensure consistency of counting data sets and could set a de facto standard on how these are counted. It will also save time since the current method includes a manual process in steps 3 and 4.

RSocrata contains the necessary functions to accomplish this task. Step 1 is using a particular read.socrata() call and step 3 can leverage ls.socrata() call. The filtering and summation can be done using base R functions.